Systems and methods for energy storage and recovery using gas expansion and compression

ABSTRACT

In various embodiments, energy-storage systems are based upon an open-air arrangement in which pressurized gas is expanded in small batches from a high pressure of, e.g., several hundred atmospheres to atmospheric pressure. The systems may be sized and operated at a rate that allows for near isothermal expansion and compression of the gas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/154,996, filed on Jun. 7, 2011, which (A) is a continuation-in-partof U.S. patent application Ser. No. 12/639,703, filed Dec. 16, 2009,which (i) is a continuation-in-part of U.S. patent application Ser. No.12/421,057, filed Apr. 9, 2009, which claims the benefit of and priorityto U.S. Provisional Patent Application No. 61/148,691, filed Jan. 30,2009, and U.S. Provisional Patent Application No. 61/043,630, filed Apr.9, 2008; (ii) is a continuation-in-part of U.S. patent application Ser.No. 12/481,235, filed Jun. 9, 2009, which claims the benefit of andpriority to U.S. Provisional Patent Application No. 61/059,964, filedJun. 9, 2008; and (iii) claims the benefit of and priority to U.S.Provisional Patent Application Nos. 61/166,448, filed on Apr. 3, 2009;61/184,166, filed on Jun. 4, 2009; 61/223,564, filed on Jul. 7, 2009;61/227,222, filed on Jul. 21, 2009; and 61/251,965, filed on Oct. 15,2009; and (B) is a continuation-in-part of U.S. patent application Ser.No. 12/938,853, filed Nov. 3, 2010, which claims the benefit of andpriority to U.S. Provisional Patent Application No. 61/257,583, filedNov. 3, 2009; U.S. Provisional Patent Application No. 61/287,938, filedDec. 18, 2009; U.S. Provisional Patent Application No. 61/310,070, filedMar. 3, 2010; and U.S. Provisional Patent Application No. 61/375,398,filed Aug. 20, 2010. The entire disclosure of each of these applicationsis hereby incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under IIP-0810590 andIIP-0923633 awarded by the NSF. The government has certain rights in theinvention.

FIELD OF THE INVENTION

In various embodiments, the present invention relates to pneumatics,hydraulics, power generation, and energy storage, and more particularly,to compressed-gas energy-storage systems and methods using pneumaticand/or hydraulic cylinders.

BACKGROUND OF THE INVENTION

As the world's demand for electric energy increases, the existing powergrid is being taxed beyond its ability to serve this demandcontinuously. In certain parts of the United States, inability to meetpeak demand has led to inadvertent brownouts and blackouts due to systemoverload and deliberate “rolling blackouts” of non-essential customersto shunt the excess demand. For the most part, peak demand occurs duringthe daytime hours (and during certain seasons, such as summer) whenbusiness and industry employ large quantities of power for runningequipment, heating, air conditioning, lighting, etc. During thenighttime hours, demand for electricity is often reduced significantly,and the existing power grid in most areas can usually handle this loadwithout problem.

To address the lack of power at peak demand, users are asked to conservewhere possible. Power companies often employ rapidly deployable gasturbines to supplement production to meet demand. However, these unitsburn expensive fuel sources, such as natural gas, and have highgeneration costs when compared with coal-fired systems, and otherlarge-scale generators. Accordingly, supplemental sources have economicdrawbacks and, in any case, can provide only a partial solution in agrowing region and economy. The most obvious solution involvesconstruction of new power plants, which is expensive and hasenvironmental side effects. In addition, because most power plantsoperate most efficiently when generating a relatively continuous output,the difference between peak and off-peak demand often leads to wastefulpractices during off-peak periods, such as over-lighting of outdoorareas, as power is sold at a lower rate off peak. Thus, it is desirableto address the fluctuation in power demand in a manner that does notrequire construction of new plants and can be implemented either at apower-generating facility to provide excess capacity during periods ofpeak demand, or on a smaller scale on-site at the facility of anelectric customer (allowing that customer to provide additional power toitself during peak demand, when the grid is over-taxed).

Another scenario in which the ability to balance the delivery ofgenerated power is highly desirable is in a self-contained generationsystem with an intermittent generation cycle. One example is a solarpanel array located remotely from a power connection. The array maygenerate well for a few hours during the day, but is nonfunctionalduring the remaining hours of low light or darkness.

In each case, the balancing of power production or provision of furthercapacity rapidly and on-demand can be satisfied by a local back-upgenerator. However, such generators are often costly, use expensivefuels, such as natural gas or diesel fuel, and are environmentallydamaging due to their inherent noise and emissions. Thus, a techniquethat allows storage of energy when not needed (such as during off-peakhours), and can rapidly deliver the power back to the user is highlydesirable.

A variety of techniques is available to store excess power for laterdelivery. One renewable technique involves the use of driven flywheelsthat are spun up by a motor drawing excess power. When the power isneeded, the flywheels' inertia is tapped by the motor or another coupledgenerator to deliver power back to the grid and/or customer. Theflywheel units are expensive to manufacture and install, however, andrequire a degree of costly maintenance on a regular basis.

Another approach to power storage is the use of batteries. Manylarge-scale batteries use a lead electrode and acid electrolyte,however, and these components are environmentally hazardous. Batteriesmust often be arrayed to store substantial power, and the individualbatteries may have a relatively short life (3-7 years is typical). Thus,to maintain a battery storage system, a large number of heavy, hazardousbattery units must be replaced on a regular basis and these oldbatteries must be recycled or otherwise properly disposed of Energy canalso be stored in ultracapacitors. A capacitor is charged by linecurrent so that it stores charge, which can be discharged rapidly whenneeded. Appropriate power-conditioning circuits are used to convert thepower into the appropriate phase and frequency of AC. However, a largearray of such capacitors is needed to store substantial electric power.Ultracapacitors, while more environmentally friendly and longer livedthan batteries, are substantially more expensive, and still requireperiodic replacement due to the breakdown of internal dielectrics, etc.

Another approach to storage of energy for later distribution involvesthe use of a large reservoir of compressed air. Storing energy in theform of compressed gas has a long history and components tend to be welltested, reliable, and have long lifetimes. The general principle ofcompressed-gas or compressed-air energy storage (CAES) is that generatedenergy (e.g., electric energy) is used to compress gas (e.g., air), thusconverting the original energy to pressure potential energy; thispotential energy is later recovered in a useful form (e.g., convertedback to electricity) via gas expansion coupled to an appropriatemechanism. Advantages of compressed-gas energy storage include lowspecific-energy costs, long lifetime, low maintenance, reasonable energydensity, and good reliability.

By way of background, a so-called compressed-air energy storage (CAES)system is shown and described in the published thesis entitled“Investigation and Optimization of Hybrid Electricity Storage SystemsBased Upon Air and Supercapacitors,” by Sylvain Lemofouet-Gatsi, EcolePolytechnique Federale de Lausanne (20 Oct. 2006) (hereafter“Lemofouet-Gatsi”), Section 2.2.1, the disclosure of which is herebyincorporated herein by reference in its entirety. As stated byLemofouet-Gatsi, “the principle of CAES derives from the splitting ofthe normal gas turbine cycle—where roughly 66% of the produced power isused to compress air—into two separated phases: The compression phasewhere lower-cost energy from off-peak base-load facilities is used tocompress air into underground salt caverns and the generation phasewhere the pre-compressed air from the storage cavern is preheatedthrough a heat recuperator, then mixed with oil or gas and burned tofeed a multistage expander turbine to produce electricity during peakdemand. This functional separation of the compression cycle from thecombustion cycle allows a CAES plant to generate three times more energywith the same quantity of fuel compared to a simple cycle natural gaspower plant.

Lemofouet-Gatsi continue, “CAES has the advantages that it doesn'tinvolve huge, costly installations and can be used to store energy for along time (more than one year). It also has a fast start-up time (9 to12 minutes), which makes it suitable for grid operation, and theemissions of greenhouse gases are lower than that of a normal gas powerplant, due to the reduced fuel consumption. The main drawback of CAES isprobably the geological structure reliance, which substantially limitsthe usability of this storage method. In addition, CAES power plants arenot emission-free, as the pre-compressed air is heated up with a fossilfuel burner before expansion. Moreover, CAES plants are limited withrespect to their effectiveness because of the loss of the compressionheat through the inter-coolers, which must be compensated duringexpansion by fuel burning. The fact that conventional CAES still rely onfossil fuel consumption makes it difficult to evaluate its energyround-trip efficiency and to compare it to conventional fuel-freestorage technologies.”

A number of variations on the above-described compressed air energystorage approach have been proposed, some of which attempt to heat theexpanded air with electricity, rather than fuel. Others employ heatexchange with thermal storage to extract and recover as much of thethermal energy as possible, therefore attempting to increaseefficiencies. Still other approaches employ compressed gas-driven pistonmotors that act both as compressors and generator drives in opposingparts of the cycle. In general, the use of highly compressed gas as aworking fluid for the motor poses a number of challenges due to thetendency for leakage around seals at higher pressures, as well as thethermal losses encountered in rapid expansion. While heat exchangesolutions can deal with some of these problems, efficiencies are stillcompromised by the need to heat compressed gas prior to expansion fromhigh pressure to atmospheric pressure.

It has been recognized that gas is a highly effective medium for storageof energy. Liquids are incompressible and flow efficiently across animpeller or other moving component to rotate a generator shaft. Oneenergy storage technique that uses compressed gas to store energy, butwhich uses a liquid, for example, hydraulic fluid, rather thancompressed gas to drive a generator, is a so-called closed-airhydraulic-pneumatic system. Such a system employs one or morehigh-pressure tanks (accumulators) having a charge of compressed gas,which is separated by a movable wall or flexible bladder membrane from acharge of hydraulic fluid. The hydraulic fluid is coupled to abi-directional impeller (or other hydraulic motor/pump), which is itselfcoupled to a combined electric motor/generator. The other side of theimpeller is connected to a low-pressure reservoir of hydraulic fluid.During a storage phase, the electric motor and impeller force hydraulicfluid from the low-pressure hydraulic fluid reservoir into thehigh-pressure tank(s), against the pressure of the compressed air. Asthe incompressible liquid fills the tank, it forces the air into asmaller space, thereby compressing it to an even higher pressure. Duringa generation phase, the fluid circuit is run in reverse and the impelleris driven by fluid escaping from the high-pressure tank(s) under thepressure of the compressed gas.

This closed-air approach has an advantage in that the gas is neverexpanded to or compressed from atmospheric pressure, as it is sealedwithin the tank. An example of a closed-air system is shown anddescribed in U.S. Pat. No. 5,579,640, the disclosure of which is herebyincorporated herein by reference in its entirety. Closed-air systemstend to have low energy densities. That is, the amount of compressionpossible is limited by the size of the tank space. In addition, sincethe gas does not completely decompress when the fluid is removed, thereis still additional energy in the system that cannot be tapped. To makea closed air system desirable for large-scale energy storage, many largeaccumulator tanks would be needed, increasing the overall cost toimplement the system and requiring more land to do so.

Another approach to hybrid hydraulic-pneumatic energy storage is theopen-air system. In this system, compressed air is stored in a large,separate high-pressure tank (or plurality of tanks). A pair ofaccumulators is provided, each having a fluid side separated from a gasside by a movable piston wall. The fluid sides of a pair (or more) ofaccumulators are coupled together through an impeller/generator/motorcombination. The air side of each of the accumulators is coupled to thehigh pressure air tanks, and also to a valve-driven atmospheric vent.Under expansion of the air chamber side, fluid in one accumulator isdriven through the impeller to generate power, and the spent fluid thenflows into the second accumulator, whose air side is now vented toatmospheric, thereby allowing the fluid to collect in the secondaccumulator. During the storage phase, electrical energy can used todirectly recharge the pressure tanks via a compressor, or theaccumulators can be run in reverse to pressurize the pressure tanks. Aversion of this open-air concept is shown and described in U.S. Pat. No.6,145,311 (the '311 patent), the disclosure of which is herebyincorporated herein by reference in its entirety. Disadvantages ofopen-air systems can include gas leakage, complexity, expense and,depending on the intended deployment, potential impracticality.

Additionally, it is desirable for solutions that address thefluctuations in power demand to also address environmental concerns andinclude using renewable energy sources. As demand for renewable energyincreases, the intermittent nature of some renewable energy sources(e.g., wind and solar) places an increasing burden on the electric grid.The use of energy storage is a key factor in addressing the intermittentnature of the electricity produced by renewable sources, and moregenerally in shifting the energy produced to the time of peak demand.

As discussed, storing energy in the form of compressed air has a longhistory. However, most of the discussed methods for converting potentialenergy in the form of compressed air to electrical energy utilizeturbines to expand the gas, which is an inherently adiabatic process. Asgas expands, it cools off if there is no input of heat (adiabatic gasexpansion), as is the case with gas expansion in a turbine. Theadvantage of adiabatic gas expansion is that it can occur quickly, thusresulting in the release of a substantial quantity of energy in a shorttime frame.

However, if the gas expansion occurs slowly relative to the time withwhich it takes for heat to flow into the gas, then the gas remains at arelatively constant temperature as it expands (isothermal gasexpansion). Gas stored at ambient temperature, which is expandedisothermally, recovers approximately three times the energy of ambienttemperature gas expanded adiabatically. Therefore, there is asignificant energy advantage to expanding gas isothermally. Gas may benot only expanded but compressed either isothermally or adiabatically.

An ideally isothermal energy-storage cycle of compression, storage, andexpansion would have 100% thermodynamic efficiency. An ideally adiabaticenergy-storage cycle would also have 100% thermodynamic efficiency, butthere are many practical disadvantages to the adiabatic approach. Theseinclude the production of more extreme temperatures and pressures withinthe system, heat loss during the storage period, and inability toexploit environmental (e.g., cogenerative) heat sources and sinks duringexpansion and compression, respectively. In an isothermal system, thecost of adding a heat-exchange system is traded against resolving thedifficulties of the adiabatic approach. In either case, mechanicalenergy from expanding gas must usually be converted to electrical energybefore use.

In the case of certain compressed gas energy storage systems accordingto prior implementations, gas is expanded from a high-pressure,high-capacity source, such as a large underground cavern, and directedthrough a multi-stage gas turbine. Because significant expansion occursat each stage of the operation, the gas cools down at each stage. Toincrease efficiency, the gas is mixed with fuel and ignited, pre-heatingit to a higher temperature, thereby increasing power and final gastemperature. However, the need to burn fossil fuel (or apply anotherenergy source, such as electric heating) to compensate for adiabaticexpansion substantially defeats the purpose of an otherwise clean andemission-free energy-storage and recovery process.

While it is technically possible to provide a direct heat-exchangesubsystem to a hydraulic/pneumatic cylinder, an external jacket, forexample, is not particularly effective given the thick walls of thecylinder. An internalized heat exchange subsystem could conceivably bemounted directly within the cylinder's pneumatic side; however, sizelimitations would reduce such a heat exchanger's effectiveness and thetask of sealing a cylinder with an added subsystem installed thereinwould be significant, and make the use of a conventional, commerciallyavailable component difficult or impossible.

Thus, the prior art does not disclose systems and methods for rapidlycompressing and expanding gas isothermally in a manner that allowsmaximum use of conventional, low-cost components, and which operates ina commercially practicable yet environmentally friendly manner.Furthermore, energy storage and recovery systems could be more morewidely deployed if they converted the work done by the linear pistonmotion directly into electrical energy or into rotary motion viamechanical means (or vice versa). In such ways, the overall efficiencyand cost-effectiveness of the compressed air system would be increased.

SUMMARY OF THE INVENTION

In various embodiments, the invention provides an energy storage system,based upon an open-air arrangement, that expands pressurized gas insmall batches from a high pressure of several hundred atmospheres toatmospheric pressure. The systems may be sized and operated at a ratethat allows for near isothermal expansion and compression of the gas.The systems may also be scalable through coupling of additionalaccumulator circuits and storage tanks as needed. Systems and methods inaccordance with the invention may allow for efficient near-isothermalhigh compression and expansion in a manner that provides a high energydensity.

Embodiments of the invention provide a system for storage and recoveryof energy using an open-air hydraulic-pneumatic accumulator andintensifier arrangement implemented in at least one circuit thatcombines an accumulator and an intensifier in communication with ahigh-pressure gas storage reservoir on the gas-side of the circuit, anda combination fluid motor/pump coupled to a combination electricgenerator/motor on the fluid side of the circuit. In a representativeembodiment, an expansion/energy recovery mode, the accumulator of afirst circuit is first filled with high-pressure gas from the reservoir,and the reservoir is then cut off from the air chamber of theaccumulator. This gas causes fluid in the accumulator to be driventhrough the motor/pump to generate electricity. Exhausted fluid isdriven into either an opposing intensifier or an accumulator in anopposing second circuit, whose air chamber is vented to atmosphere. Asthe gas in the accumulator expands to mid-pressure, and fluid isdrained, the mid-pressure gas in the accumulator is then connected to anintensifier with a larger-area air piston acting on a smaller area fluidpiston. Fluid in the intensifier is then driven through the motor/pumpat still-high fluid pressure, despite the mid-pressure gas in theintensifier air chamber. Fluid from the motor/pump is exhausted intoeither the opposing first accumulator or an intensifier of the secondcircuit, whose air chamber may be vented to atmosphere as thecorresponding fluid chamber fills with exhausted fluid. In acompression/energy storage stage, the process is reversed and the fluidmotor/pump is driven by the electric component to force fluid into theintensifier and the accumulator to compress gas and deliver it to thetank reservoir under high pressure.

Embodiments of the present invention also obviate the need for ahydraulic subsystem by converting the reciprocal motion of energystorage and recovery cylinders into electrical energy via alternativemeans. In some embodiments, the invention combines a compressed-gasenergy storage system with a linear-generator system for the generationof electricity from reciprocal motion to increase system efficiency andcost-effectiveness. The same arrangement of devices may be used toconvert electric energy to potential energy in compressed gas, withsimilar gains in efficiency and cost-effectiveness.

Another alternative, utilized in various embodiments, to the use ofhydraulic fluid to transmit force between the motor/generator and thegas undergoing compression or expansion is the mechanical transmissionof the force. In particular, the linear motion of the cylinder piston orpistons may be coupled to a crankshaft or other means of conversion torotary motion. The crankshaft may in turn be coupled to, e.g., a gearbox or a continuously variable transmission (CVT) that drives the shaftof an electric motor/generator at a rotational speed higher than that ofthe crankshaft. The continuously variable transmission, within itsoperable range of effective gear ratios, allows the motor/generator tobe operated at constant speed regardless of crankshaft speed. Themotor/generator operating point can be chosen for optimal efficiency;constant output power is also desirable. Multiple pistons may be coupledto a single crankshaft, which may be advantageous for purposes of shaftbalancing.

The power output of these systems is governed by how fast the gas canexpand isothermally. Therefore, the ability to expand/compress the gasisothermally at a faster rate will result in a greater power output ofthe system. By adding a heat transfer subsystem to these systems, thepower density of said system may be increased substantially. Therefore,energy storage and generation systems in accordance with embodiments ofthe invention include a heat-transfer subsystem for expediting heattransfer in one or more compartments of the cylinder assembly. In oneembodiment, the heat-transfer subsystem includes a fluid circulator anda heat-transfer fluid reservoir. The fluid circulator pumps aheat-transfer fluid into the first compartment and/or the secondcompartment of the pneumatic cylinder. The heat-transfer subsystem mayalso include a spray mechanism, disposed in the first compartment and/orthe second compartment, for introducing the heat-transfer fluid. Invarious embodiments, the spray mechanism is a spray head and/or a sprayrod.

Gas undergoing expansion tends to cool, while gas undergoing compressiontends to heat. To maximize efficiency (i.e., the fraction of elasticpotential energy in the compressed gas that is converted to work, orvice versa), gas expansion and compression should be as near isothermal(i.e., constant-temperature) as possible. Several ways of approximatingisothermal expansion and compression may be employed.

First, droplets of a liquid (e.g., water) may be sprayed into a chamberof the pneumatic cylinder in which gas is presently undergoingcompression (or expansion) in order to transfer heat to or from the gas.As the liquid droplets exchange heat with the gas around them, thetemperature of the gas is raised or lowered; the temperature of thedroplets is also raised or lowered. The liquid is evacuated from thecylinder through a suitable mechanism. The heat-exchange spray dropletsmay be introduced through a spray head (in, e.g., a vertical cylinder),through a spray rod arranged coaxially with the cylinder piston (in,e.g., a horizontal cylinder), or by any other mechanism that permitsformation of a liquid spay within the cylinder. Droplets may be used toeither warm gas undergoing expansion or to cool gas undergoingcompression. An isothermal process may be approximated via judiciousselection of this heat-exchange rate.

Furthermore, as described in U.S. Pat. No. 7,802,426 (the '426 patent),the disclosure of which is hereby incorporated by reference herein inits entirety, gas undergoing either compression or expansion may bedirected, continuously or in installments, through a heat-exchangesubsystem external to the cylinder. The heat-exchange subsystem eitherrejects heat to the environment (to cool gas undergoing compression) orabsorbs heat from the environment (to warm gas undergoing expansion).Again, an isothermal process may be approximated via judicious selectionof this heat-exchange rate.

As mentioned above, some embodiments of the present invention utilize alinear motor/generator as an alternative to the conventional rotarymotor/generator. Like a rotary motor/generator, a linearmotor/generator, when operated as a generator, converts mechanical powerto electrical power by exploiting Faraday's law of induction: that is,the magnetic flux through a closed circuit is made to change by moving amagnet, thus inducing an electromotive force (EMF) in the circuit. Thesame device may also be operated as a motor.

There are several forms of linear motor/generator, but for simplicity,the discussion herein mainly pertains to the permanent-magnet tubulartype. In some applications tubular linear generators have advantagesover flat topologies, including smaller leakage, smaller coils withconcomitant lower conductor loss and higher force-to-weight ratio. Forbrevity, only operation in generator mode is described herein. Theability of such a machine to operate as either a motor or generator willbe apparent to any person reasonably familiar with the principles ofelectrical machines.

In a typical tubular linear motor/generator, permanentradially-magnetized magnets, sometimes alternated with iron core rings,are affixed to a shaft. The permanent magnets have alternatingmagnetization. This armature, composed of shaft and magnets, is termed atranslator or mover and moves axially through a tubular winding orstator. Its function is analogous to that of a rotor in a conventionalgenerator. Moving the translator through the stator in either directionproduces a pulse of alternating EMF in the stator coil. The tubularlinear generator thus produces electricity from a source ofreciprocating motion. Moreover, such generators offer the translation ofsuch mechanical motion into electrical energy with high efficiency,since they obviate the need for gear boxes or other mechanisms toconvert reciprocal into rotary motion. Since a linear generator producesa series of pulses of alternating current (AC) power with significantharmonics, power electronics are typically used to condition the outputof such a generator before it is fed to the power grid. However, suchpower electronics require less maintenance and are less prone to failurethan the mechanical linear-to-rotary conversion systems which wouldotherwise be required. Operated as a motor, such a tubular linearmotor/generator produces reciprocating motion from an appropriateelectrical excitation.

In compressed-gas energy storage systems in accordance with embodimentsof the present invention, gas is stored at high pressure (e.g.,approximately 3000 pounds per square inch gauge (psig)). This gas isexpanded into a chamber of a cylinder containing a piston or othermechanism that separates the gas on one side of the cylinder from theother, preventing gas movement from one chamber to the other whileallowing the transfer of force/pressure from one chamber to the next.The shaft of the cylinder may be attached to a mechanical load, e.g.,the translator of a linear generator. In the simplest arrangement, thecylinder shaft and translator are in line (i.e., aligned on a commonaxis). In some embodiments, the shaft of the cylinder is coupled to atransmission mechanism for converting a reciprocal motion of the shaftinto a rotary motion, and a motor/generator is coupled to thetransmission mechanism. In some embodiments, the transmission mechanismincludes a crankshaft and a gear box. In other embodiments, thetransmission mechanism includes a crankshaft and a CVT. A CVT is atransmission that can move smoothly through a continuum of effectivegear ratios over some finite range.

In various embodiments described herein, reciprocal motion is producedduring recovery of energy from storage by expansion of gas in pneumaticcylinders. In various embodiments, this reciprocal motion is convertedto rotary motion by first using the expanding gas to drive apneumatic/hydraulic intensifier; the hydraulic fluid pressurized by theintensifier drives a hydraulic rotary motor/generator to produceelectricity. (The system is run in reverse to convert electric energyinto potential energy in compressed gas.) By mechanically couplinglinear generators to pneumatic cylinders, the hydraulic system may beomitted, typically with increased efficiency and reliability.Conversely, a linear motor/generator may be operated as a motor in orderto compress gas in pneumatic cylinders for storage in a reservoir. Inthis mode of operation, the device converts electrical energy tomechanical energy rather than the reverse. The potential advantages ofusing a linear electrical machine may thus accrue to both the storageand recovery operations of a compressed-gas energy storage system.

In various embodiments, the compression and expansion occurs in multiplestages, using low- and high-pressure cylinders. For example, inexpansion, high-pressure gas is expanded in a high-pressure cylinderfrom a maximum pressure (e.g., approximately 3,000 psig) to somemid-pressure (e.g. approximately 300 psig); then this mid-pressure gasis further expanded further (e.g., approximately 300 psig toapproximately 30 psig) in a separate low-pressure cylinder. Thus, ahigh-pressure cylinder may handle a maximum pressure up to approximatelya factor of ten greater than that of a low-pressure cylinder.Furthermore, the ratio of maximum to minimum pressure handled by ahigh-pressure cylinder may be approximately equal to ten (or evengreater), and/or may be approximately equal to such a ratio of thelow-pressure cylinder. The minimum pressure handled by a high-pressurecylinder may be approximately equal to the maximum pressure handled by alow-pressure cylinder.

The two stages may be tied to a common shaft and driven by a singlelinear motor/generator (or may be coupled to a common crankshaft, asdetailed below). When each piston reaches the limit of its range ofmotion (e.g., reaches the end of the low-pressure side of the chamber),valves or other mechanisms may be adjusted to direct gas to theappropriate chambers. In double-acting devices of this type, there is nowithdrawal stroke or unpowered stroke: the stroke is powered in bothdirections.

Since a tubular linear generator is inherently double-acting (i.e.,generates power regardless of which way the translator moves), theresulting system generates electrical power at all times other than whenthe piston is hesitating between strokes. Specifically, the output ofthe linear generator may be a series of pulses of AC power, separated bybrief intervals of zero power output during which the mechanism reversesits stroke direction. Power electronics may be employed with short-termenergy storage devices such as ultracapacitors to condition thiswaveform to produce power acceptable for the grid. Multiple unitsoperating out-of-phase may also be used to minimize the need forshort-term energy storage during the transition periods of individualgenerators.

Use of a CVT enables the motor/generator to be operated at constanttorque and speed over a range of crankshaft rotational velocities. Theresulting system generates electrical power continuously and at a fixedoutput level as long as pressurized air is available from the reservoir.As mentioned above, power electronics and short-term energy storagedevices such as ultracapacitors may, if needed, condition the waveformproduced by the motor/generator to produce power acceptable for thegrid.

In various embodiments, the system also includes a source of compressedgas and a control-valve arrangement for selectively connecting thesource of compressed gas to an input of the first compartment (or“chamber”) of the pneumatic cylinder assembly and an input of the secondcompartment of the pneumatic cylinder assembly. The system may alsoinclude a second pneumatic cylinder assembly having a first compartmentand a second compartment separated by a piston slidably disposed withinthe cylinder and a shaft coupled to the piston and extending through atleast one of the first compartment and the second compartment of thesecond cylinder and beyond an end cap of the second cylinder and coupledto a transmission mechanism. The second pneumatic cylinder assembly maybe fluidly coupled to the first pneumatic cylinder assembly. Forexample, the pneumatic cylinder assemblies may be coupled in series.Additionally, one of the pneumatic cylinder assemblies may be ahigh-pressure cylinder and the other pneumatic cylinder assembly may bea low-pressure cylinder. The low-pressure cylinder assembly may bevolumetrically larger, e.g., may have an interior volume at least 50%larger, than the high-pressure cylinder assembly.

A further opportunity for increased efficiency arises from the fact thatas gas in the high-pressure storage vessel is exhausted, its pressuredecreases. Thus, in order to extract as much energy as possible from agiven quantity of stored gas, the electricity-producing side of such anenergy-storage system must operate over a wide range of input pressures,i.e., from the reservoir's high-pressure limit (e.g., approximately3,000 psig) to as close to atmospheric pressure as possible. At lowerpressure, gas expanding in a cylinder exerts a smaller force on itspiston and thus on the translator of the linear generator (or to therotor of the generator) to which it is coupled. For a fixed pistonspeed, this generally results in reduced power output.

In various embodiments, however, power output is substantially constant.Constant power may be maintained with decreased force by increasingpiston linear speed. Piston speed may be regulated, for example, byusing power electronics to adjust the electrical load on a lineargenerator so that translator velocity is increased (with correspondinglyhigher voltage and lower current induced in the stator) as the pressureof the gas in the high-pressure storage vessel decreases. At lowergas-reservoir pressures, in such an arrangement, the pulses of AC powerproduced by the linear generator will be shorter in duration and higherin frequency, requiring suitable adjustments in the power electronics tocontinue producing grid-suitable power.

With variable linear motor/generator speed, efficiency gains may berealized by using variable-pitch windings and/or a switched-reluctancelinear generator. In a switched-reluctance generator, the mover (i.e.,translator or rotor) contains no permanent magnets; rather, magneticfields are induced in the mover by windings in the stator which arecontrolled electronically. The position of the mover is either measuredor calculated, and excitement of the stator windings is electronicallyadjusted in real time to produce the desired torque (or traction) forany given mover position and velocity.

Substantially constant power may also be achieved by mechanical linkageswhich vary the torque for a given force. Other techniques include pistonspeed regulation by using power electronics to adjust the electricalload on the motor/generator so that crankshaft velocity is increased,which for a fixed torque will increase power. For such arrangementsusing power electronics, the center frequency and harmonics of the ACwaveform produced by the motor/generator typically change, which mayrequire suitable adjustments in the power electronics to continueproducing grid-suitable power.

Use of a CVT to couple a crankshaft to a motor/generator is yet anotherway to achieve approximately constant power output in accordance withembodiments of the invention. Generally, there are two challenges to themaintenance of constant output power. First is the discrete pistonstroke. As a quantity of gas is expanded in a cylinder during the courseof a single stroke, its pressure decreases; to maintain constant poweroutput from the cylinder as the force acting on its piston decreases,the piston's linear velocity is continually increased throughout thestroke. This increases the crankshaft angular velocity proportionatelythroughout the stroke. To maintain constant angular velocity andconstant power at the input shaft of the motor/generator throughout thestroke, the effective gear ratio of the CVT is adjusted continuously tooffset increasing crankshaft speed.

Second, pressure in the main gas store decreases as the store isexhausted. As this occurs, the piston velocity at all points along thestroke is typically increased to deliver constant power. Crankshaftangular velocity is therefore also typically increased at all times.

Under these illustrative conditions, the effective gear ratio of the CVTthat produces substantially constant output power, plotted as a functionof time, has the approximate form of a periodic sawtooth (correspondingto CVT adjustment during each discrete stroke) superimposed on a ramp(corresponding to CVT adjustment compensating for exhaustion of the gasstore.)

With either a linear or rotary motor/generator, the range of forces (andthus of speeds) is generally minimized in order to achieve maximizeefficiency. In lieu of more complicated linkages, for a given operatingpressure range (e.g., from approximately 3,000 psig to approximately 30psig), the range of forces (torques) seen at the motor/generator may bereduced through the addition of multiple cylinder stages arranged, e.g.,in series. That is, as gas from the high-pressure reservoir is expandedin one chamber of an initial, high-pressure cylinder, gas from the otherchamber is directed to the expansion chamber of a second, lower-pressurecylinder. Gas from the lower-pressure chamber of this second cylindermay either be vented to the environment or directed to the expansionchamber of a third cylinder operating at still lower pressure, and soon. An arrangement using two cylinder assemblies is shown and described;however, the principle may be extended to more than two cylinders tosuit a particular application.

For example, a narrower force range over a given range of reservoirpressures is achieved by having a first, high-pressure cylinderoperating between approximately 3,000 psig and approximately 300 psigand a second, larger-volume, low-pressure cylinder operating betweenapproximately 300 psig and approximately 30 psig. The range of pressures(and thus of force) is reduced as the square root, from 100:1 to 10:1,compared to the range that would be realized in a single cylinderoperating between approximately 3,000 psig and approximately 30 psig.The square-root relationship between the two-cylinder pressure range andthe single-cylinder pressure range can be demonstrated as follows.

A given pressure range R₁ from high pressure P_(H) to low pressureP_(L), namely R_(I)=P_(H)/P_(L), is subdivided into two pressure rangesof equal magnitude R₂. The first range is from P_(H) down to someintermediate pressure P_(I) and the second is from P_(I) down to P_(L).Thus, R₂=P_(H)/P_(I)=P_(I)/P_(L). From this identity of ratios,P_(I)=(P_(H)P_(L))^(1/2). Substituting for P_(I) in R₂=P_(H)/P_(I), weobtain R₂P_(H)/(P_(H)P_(L))^(1/2)=(P_(H)P_(L))^(1/2)=R₁ ^(1/2). It maybe similarly shown that with appropriate cylinder sizing, the additionof a third cylinder/stage reduces the operating pressure range as thecube root, and so forth. In general (and as also set forth herein), Nappropriately sized cylinders reduce an original (i.e., single-cylinder)operating pressure range R₁ to R₁ ^(1/N). Any group of N cylindersstaged in this manner, where N≧2, is herein termed a cylinder group.

In various embodiments, the shafts of two or more double-actingcylinders are connected either to separate linear motor/generators or toa single linear motor/generator, either in line or in parallel. If theyare connected in line, their common shaft may be arranged in line withthe translator of a linear motor/generator. If they are connected inparallel, their separate shafts may be linked to a transmission (e.g.,rigid beam) that is orthogonal to the shafts and to the translator ofthe motor/generator. Another portion of the beam may be attached to thetranslator of a linear generator that is aligned in parallel with thetwo cylinders. The synchronized reciprocal motion of the twodouble-acting cylinders may thus be transmitted to the linear generator.

In other embodiments of the invention, two or more cylinder groups,which may be identical, may be coupled to a common crankshaft. Acrosshead arrangement may be used for coupling each of the N pneumaticcylinder shafts in each cylinder group to the common crankshaft. Thecrankshaft may be coupled to an electric motor/generator either directlyor via a gear box. If the crankshaft is coupled directly to an electricmotor/generator, the crankshaft and motor/generator may turn at very lowspeed (very low revolutions per minute, RPM), e.g., 25-30 RPM, asdetermined by the cycle speed of the cylinders.

Any multiple-cylinder implementation of this invention such as thatdescribed above may be co-implemented with any of the heat-transfermechanisms described earlier.

All of the mechanisms described herein for converting potential energyin compressed gas to electrical energy, including the heat-exchangemechanisms and power electronics described, can, if appropriatelydesigned, be operated in reverse to store electrical energy as potentialenergy in a compressed gas. Since this will be apparent to any personreasonably familiar with the principles of electrical machines, powerelectronics, pneumatics, and the principles of thermodynamics, theoperation of these mechanisms to store energy rather than to recover itfrom storage will not be described in many embodiments. Such operationis, however, contemplated and within the scope of the invention and maybe straightforwardly realized without undue experimentation.

In an aspect, embodiments of the invention feature an energy storage andgeneration system including or consisting essentially of a firstpneumatic cylinder assembly for compressing gas to store energy and/orexpanding gas to recover energy, a motor/generator outside the firstcylinder assembly, a transmission mechanism, a heat-transfer subsystem,and a control system for controlling operation of the first pneumaticcylinder assembly to enforce substantially isothermal expansion andcompression of gas therein to thereby increase efficiency of theexpansion and compression. The first cylinder assembly includes orconsists essentially of a first compartment, a second compartment, and apiston separating the compartments. The transmission mechanism iscoupled to the piston and the motor/generator and converts reciprocalmotion of the piston into rotary motion of the motor/generator and/orconverts rotary motion of the motor/generator into reciprocal motion ofthe piston. The heat-transfer subsystem expedites heat transfer in thefirst compartment and/or the second compartment of the first pneumaticcylinder assembly. The control system is responsive to at least onesystem parameter associated with operation of the first pneumaticcylinder assembly.

Embodiments of the invention may include one or more of the following,in any of a variety of combinations. The system may include a shafthaving a first end coupled to the piston and a second end coupled to thetransmission mechanism (e.g., by a crosshead linkage). The system mayinclude a container for storage of compressed gas after compressionand/or supply of compressed gas for expansion thereof, as well as anarrangement for selectively permitting fluid communication of thecontainer with at least one compartment of the first pneumatic cylinderassembly. A second pneumatic cylinder assembly, including or consistingessentially of a first compartment, a second compartment, and a pistonseparating the compartments (and coupled to the transmission mechanism),may be fluidly coupled to the first pneumatic cylinder assembly (e.g.,in series). The second pneumatic cylinder assembly may include a shafthaving a first end coupled to the piston of the second pneumaticcylinder assembly and a second end coupled to the transmission mechanism(e.g., by a crosshead linkage).

The transmission mechanism may include or consist essentially of acrankshaft, a crankshaft and a gear box, or a crankshaft and acontinuously variable transmission. The heat-transfer subsystem mayinclude a fluid circulator for pumping heat-transfer liquid into thefirst compartment and/or the second compartment of the first pneumaticcylinder assembly. A mechanism for introducing the heat-transfer fluid(e.g., a spray head and/or a spray rod) may be disposed in the firstcompartment and/or the second compartment of the first pneumaticcylinder assembly. The transmission mechanism may vary torque for agiven force exerted on the transmission mechanism. The system mayinclude power electronics for adjusting a load on the motor/generator.The at least one system parameter may include or consist essentially ofa fluid state, a fluid flow, a temperature, and/or a pressure. Thesystem may include one or more sensors that monitor the at least onesystem parameter, and the control system may be responsive to thesensor(s). The system may include a vent for supply of gas forcompression and/or exhausting gas after expansion. Energy stored duringcompression of gas may originate from an intermittent renewable energysource (e.g., of wind or solar energy). Energy may be recovered viaexpansion of gas when the intermittent renewable energy source isnonfunctional.

These and other objects, along with the advantages and features of thepresent invention herein disclosed, will become apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and canexist in various combinations and permutations. Herein, the terms“liquid” and “water” interchangeably connote any mostly or substantiallyincompressible liquid, the terms “gas” and “air” are usedinterchangeably, and the term “fluid” may refer to a liquid or a gasunless otherwise indicated. As used herein, the term “substantially”means±10%, and, in some embodiments, ±5%. A “valve” is any mechanism orcomponent for controlling fluid communication between fluid paths orreservoirs, or for selectively permitting control or venting. The term“cylinder” refers to a chamber, of uniform but not necessarily circularcross-section, which may contain a slidably disposed piston or othermechanism that separates the fluid on one side of the chamber from thaton the other, preventing fluid movement from one side of the chamber tothe other while allowing the transfer of force/pressure from one side ofthe chamber to the next or to a mechanism outside the chamber. In theabsence of a mechanical separation mechanism, a “chamber” or“compartment” of a cylinder may correspond to substantially the entirevolume of the cylinder. A “cylinder assembly” may be a simple cylinderor include multiple cylinders, and may or may not have additionalassociated components (such as mechanical linkages among the cylinders).

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. In addition, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1 is a schematic diagram of an open-air hydraulic-pneumatic energystorage and recovery system in accordance with one embodiment of theinvention;

FIGS. 1A and 1B are enlarged schematic views of the accumulator andintensifier components of the system of FIG. 1;

FIGS. 2A-2Q are simplified graphical representations of the system ofFIG. 1 illustrating the various operational stages of the system duringcompression;

FIGS. 3A-3M are simplified graphical representations of the system ofFIG. 1 illustrating the various operational stages of the system duringexpansion;

FIG. 4, incorporating, as shown, partial views FIG. 4A and FIG. 4B, is aschematic diagram of an open-air hydraulic-pneumatic energy storage andrecovery system in accordance with an alternative embodiment of theinvention;

FIGS. 5A-5N are schematic diagrams of the system of FIG. 4 illustratingthe cycling of the various components during an expansion phase of thesystem;

FIG. 6 is a generalized diagram of the various operational states of anopen-air hydraulic-pneumatic energy storage and recovery system inaccordance with one embodiment of the invention in both anexpansion/energy recovery cycle and a compression/energy storage cycle;

FIGS. 7A-7F are partial schematic diagrams of an open-airhydraulic-pneumatic energy storage and recovery system in accordancewith another alternative embodiment of the invention, illustrating thevarious operational stages of the system during an expansion phase;

FIG. 8 is a table illustrating the expansion phase for the system ofFIGS. 7A-7F;

FIG. 9 is a schematic diagram of an open-air hydraulic-pneumatic energystorage and recovery system including a heat transfer subsystem inaccordance with one embodiment of the invention;

FIG. 9A is an enlarged schematic diagram of the heat transfer subsystemportion of the system of FIG. 9;

FIG. 10 is a graphical representation of the thermal efficienciesobtained by the system of FIG. 9 at different operating parameters;

FIG. 11 is a schematic partial cross section of a hydraulic/pneumaticcylinder assembly including a heat transfer subsystem that facilitiesisothermal expansion within the pneumatic side of the cylinder inaccordance with one embodiment of the invention;

FIG. 12 is a schematic partial cross section of a hydraulic/pneumaticintensifier assembly including a heat transfer subsystem that facilitiesisothermal expansion within the pneumatic side of the cylinder inaccordance with an alternative embodiment of the invention;

FIG. 13 is a schematic partial cross section of a hydraulic/pneumaticcylinder assembly having a heat transfer subsystem that facilitatesisothermal expansion within the pneumatic side of the cylinder inaccordance with another alternative embodiment of the invention in whichthe cylinder is part of a power generating system;

FIG. 14A is a graphical representation of the amount of work producedbased upon an adiabatic expansion of gas within the pneumatic side of acylinder or intensifier for a given pressure versus volume;

FIG. 14B is a graphical representation of the amount of work producedbased upon an ideal isothermal expansion of gas within the pneumaticside of a cylinder or intensifier for a given pressure versus volume;

FIG. 14C is a graphical representation of the amount of work producedbased upon a near-isothermal expansion of gas within the pneumatic sideof a cylinder or intensifier for a given pressure versus volume;

FIG. 15 is a schematic diagram of a system and method for expedited heattransfer to gas expanding (or being compressed) in an open-air stagedhydraulic-pneumatic system in accordance with one embodiment of theinvention;

FIG. 16 is a schematic diagram of a system and method for expedited heattransfer to gas expanding (or being compressed) in an open-air stagedhydraulic-pneumatic system in accordance with another embodiment of theinvention;

FIG. 17 is a schematic diagram of a system and method for expedited heattransfer to gas expanding (or being compressed) in an open-air stagedhydraulic-pneumatic system in accordance with yet another embodiment ofthe invention;

FIG. 18 is a schematic diagram of a system and method for expedited heattransfer to gas expanding (or being compressed) in an open-air stagedhydraulic-pneumatic system in accordance with another embodiment of theinvention;

FIG. 19 is a schematic diagram of a system and method for expedited heattransfer to gas expanding (or being compressed) in an open-air stagedhydraulic-pneumatic system in accordance with another embodiment of theinvention;

FIGS. 20A and 20B are schematic diagrams of a system and method forexpedited heat transfer to gas expanding (or being compressed) in anopen-air staged hydraulic-pneumatic system in accordance with anotherembodiment of the invention;

FIGS. 21A-21C are schematic diagrams of a system and method forexpedited heat transfer to gas expanding (or being compressed) in anopen-air staged hydraulic-pneumatic system in accordance with anotherembodiment of the invention;

FIGS. 22A and 22B are schematic diagrams of a system and method forexpedited heat transfer to gas expanding (or being compressed) in anopen-air staged hydraulic-pneumatic system in accordance with anotherembodiment of the invention;

FIG. 22C is a schematic cross-sectional view of a cylinder assembly foruse in the system and method of FIGS. 22A and 22B;

FIG. 22D is a graphical representation of the estimated water spray heattransfer limits for an implementation of the system and method of FIGS.22A and 22B;

FIGS. 23A and 23B are schematic diagrams of a system and method forexpedited heat transfer to gas expanding (or being compressed) in anopen-air staged hydraulic-pneumatic system in accordance with anotherembodiment of the invention;

FIG. 23C is a schematic cross-sectional view of a cylinder assembly foruse in the system and method of FIGS. 23A and 23B;

FIG. 23D is a graphical representation of the estimated water spray heattransfer limits for an implementation of the system and method of FIGS.23A and 23B;

FIGS. 24A and 24B are graphical representations of the various waterspray requirements for the systems and methods of FIGS. 22 and 23;

FIG. 25 is a detailed schematic plan view in partial cross-section of acylinder design for use in any of the foregoing embodiments of theinvention described herein for expedited heat transfer to gas expanding(or being compressed) in an open-air staged hydraulic-pneumatic systemin accordance with one embodiment of the invention;

FIG. 26 is a detailed schematic plan view in partial cross-section of acylinder design for use in any of the foregoing embodiments of theinvention described herein for expedited heat transfer to gas expanding(or being compressed) in an open-air staged hydraulic-pneumatic systemin accordance with one embodiment of the invention;

FIG. 27 is a schematic diagram of a compressed-gas storage subsystem foruse with systems and methods for heating and cooling compressed gas inenergy storage systems in accordance with one embodiment of theinvention;

FIG. 28 is a schematic diagram of a compressed-gas storage subsystem foruse with systems and methods for heating and cooling of compressed gasfor energy storage systems in accordance with an alternative embodimentof the invention;

FIGS. 29A and 29B are schematic diagrams of a staged hydraulic-pneumaticenergy conversion system including a heat transfer subsystem inaccordance with one embodiment of the invention;

FIGS. 30A-30D are schematic diagrams of a staged hydraulic-pneumaticenergy conversion system including a heat transfer subsystem inaccordance with an alternative embodiment of the invention;

FIGS. 31A-31C are schematic diagrams of a staged hydraulic-pneumaticenergy conversion system including a heat transfer subsystem inaccordance with another alternative embodiment of the invention;

FIG. 32 is a schematic cross-sectional diagram showing the use ofpressurized stored gas to operate a double-acting pneumatic cylinder anda linear motor/generator to produce electricity or stored pressurizedgas according to various embodiments of the invention;

FIG. 33 depicts the mechanism of FIG. 32 in a different phase ofoperation (i.e., with the high- and low-pressure sides of the pistonreversed and the direction of shaft motion reversed);

FIG. 34 depicts the arrangement of FIG. 32 modified to introduce liquidsprays into the two compartments of the cylinder, in accordance withvarious embodiments of the invention;

FIG. 35 depicts the mechanism of FIG. 34 in a different phase ofoperation (i.e., with the high- and low-pressure sides of the pistonreversed and the direction of shaft motion reversed);

FIG. 36 depicts the mechanism of FIG. 32 modified by the addition of anexternal heat exchanger in communication with both compartments of thecylinder, where the contents of either compartment may be circulatedthrough the heat exchanger to transfer heat to or from the gas as itexpands or compresses, enabling substantially isothermal expansion orcompression of the gas, in accordance with various embodiments of theinvention;

FIG. 37 depicts the mechanism of FIG. 32 modified by the addition of asecond pneumatic cylinder operating at a lower pressure than the first,in accordance with various embodiments of the invention;

FIG. 38 depicts the mechanism of FIG. 37 in a different phase ofoperation (i.e., with the high- and low-pressure sides of the pistonsreversed and the direction of shaft motion reversed);

FIG. 39 depicts the mechanism of FIG. 32 modified by the addition of asecond pneumatic cylinder operating at lower pressure, in accordancewith various embodiments of the invention;

FIG. 40 depicts the mechanism of FIG. 39 in a different phase ofoperation (i.e., with the high- and low-pressure sides of the pistonsreversed and the direction of shaft motion reversed);

FIG. 41 is a schematic diagram of a system and related method forsubstantially isothermal compression and expansion of a gas for energystorage using one or more pneumatic cylinders in accordance with variousembodiments of the invention;

FIG. 42 is a schematic diagram of the system of FIG. 41 in a differentphase of operation;

FIG. 43 is a schematic diagram of a system and related method forcoupling a cylinder shaft to a crankshaft; and

FIGS. 44A and 44B are schematic diagrams of systems in accordance withvarious embodiments of the invention, in which multiple cylinder groupsare coupled to a single crankshaft.

DETAILED DESCRIPTION

In the following, various embodiments of the present invention aregenerally described with reference to a single accumulator and a singleintensifier or an arrangement with two accumulators and two intensifiersand simplified valve arrangements. It is, however, to be understood thatthe present invention can include any number and combination ofaccumulators, intensifiers, and valve arrangements. In addition, anydimensional values given are exemplary only, as the systems according tothe invention are scalable and customizable to suit a particularapplication. Furthermore, the terms pneumatic, gas, and air are usedinterchangeably and the terms hydraulic, fluid, and liquid are also usedinterchangeably.

FIG. 1 depicts one embodiment of an open-air hydraulic-pneumatic energystorage and recovery system 100 in accordance with the invention in aneutral state (i.e., all of the valves are closed and energy is neitherbeing stored nor recovered. The system 100 includes one or morehigh-pressure gas/air storage tanks 102 a, 102 b, . . . 102 n. In FIG. 1and other figures herein, wherever a series of n objects is referred to,only a definite number of objects (e.g., two) may be explicitlydepicted. Each tank 102 is joined in parallel via a manual valve(s) 104a, 104 b, . . . 104 n, respectively, to a main air line 108. The valves104 are not limited to manual operation, but can be electrically,hydraulically, or pneumatically actuated, as can all of the valvesdescribed herein. The tanks 102 are each provided with a pressure sensor112 a, 112 b 112 n and a temperature sensor 114 a, 114 b . . . 114 n.These sensors 112, 114 can output electrical signals that can bemonitored by a control system 120 via appropriate wired and wirelessconnections/communications. Additionally, the sensors 112, 114 couldinclude visual indicators.

The control system 120, which is described in greater detail withrespect to FIG. 4, can be any acceptable control device with ahuman-machine interface. For example, the control system 120 couldinclude a computer (for example a PC-type) that executes a storedcontrol application in the form of a computer-readable software medium.The control application receives telemetry from the various sensors tobe described below, and provides appropriate feedback to control valveactuators, motors, and other needed electromechanical/electronicdevices.

The system 100 further includes pneumatic valves 106 a, 106 b, 106 c, .. . 106 n that control the communication of the main air line 108 withan accumulator 116 and an intensifier 118. As previously stated, thesystem 100 can include any number and combination of accumulators 116and intensifiers 118 to suit a particular application. The pneumaticvalves 106 are also connected to a vent 110 for exhausting air/gas fromthe accumulator 116, the intensifier 118, and/or the main air line 108.

As shown in FIG. 1A, the accumulator 116 includes an air chamber 140 anda fluid chamber 138 divided by a movable piston 136 having anappropriate sealing system using sealing rings and other components (notshown) that are known to those of ordinary skill in the art.Alternatively, a bladder type barrier could be used to divide the airand fluid chambers 140, 138 of the accumulator 116. The piston 136 movesalong the accumulator housing in response to pressure differentialsbetween the air chamber 140 and the opposing fluid chamber 138. In thisexample, hydraulic fluid (or another liquid, such as water) is indicatedby a partially shaded volume in the fluid chamber 138. The accumulator116 can also include optional shut-off valves 134 that can be used toisolate the accumulator 116 from the system 100. The valves 134 can bemanually or automatically operated.

As shown in FIG. 1B, the intensifier 118 includes an air chamber 144 anda fluid chamber 146 divided by a movable piston assembly 142 having anappropriate sealing system using sealing rings and other components thatare known to those of ordinary skill in the art. Similar to theaccumulator piston 136, the intensifier piston 142 moves along theintensifier housing in response to pressure differentials between theair chamber 144 and the opposing fluid chamber 146.

However, the intensifier piston assembly 142 is actually two pistons: anair piston 142 a connected by a shaft, rod, or other coupling means 143to a respective fluid piston 142 b. The fluid piston 142 b moves inconjunction with the air piston 142 a, but acts directly upon theassociated intensifier fluid chamber 146. Notably, the internal diameter(and/or volume) (DAI) of the air chamber for the intensifier 118 isgreater than the diameter (DAA) of the air chamber for the accumulator116. In particular, the surface of the intensifier piston 142 a isgreater than the surface area of the accumulator piston 136. Thediameter of the intensifier fluid piston (DFI) is approximately the sameas the diameter of the accumulator piston 136 (DFA). Thus in thismanner, a lower air pressure acting upon the intensifier piston 142 agenerates a similar pressure on the associated fluid chamber 146 as ahigher air pressure acting on the accumulator piston 136. As such, theratio of the pressures of the intensifier air chamber 144 and theintensifier fluid chamber 146 is greater than the ratio of the pressuresof the accumulator air chamber 140 and the accumulator fluid chamber138. In one example, the ratio of the pressures in the accumulator couldbe 1:1, while the ratio of pressures in the intensifier could be 10:1.These ratios will vary depending on the number of accumulators andintensifiers used and the particular application. In this manner, and asdescribed further below, the system 100 allows for at least two stagesof air pressure to be employed to generate similar levels of fluidpressure. Again, a shaded volume in the fluid chamber 146 indicates thehydraulic fluid and the intensifier 118 can also include the optionalshut-off valves 134 to isolate the intensifier 118 from the system 100.

As also shown in FIGS. 1A and 1B, the accumulator 116 and theintensifier 118 each include a temperature sensor 122 and a pressuresensor 124 in communication with each air chamber 140, 144 and eachfluid chamber 138, 146. These sensors are similar to sensors 112, 114and deliver sensor telemetry to the control system 120, which in turncan send signals to control the valve arrangements. In addition, thepistons 136, 142 can include position sensors 148 that report thepresent position of the pistons 136, 142 to the control system 120. Theposition and/or rate of movement of the pistons 136, 142 can be used todetermine relative pressure and flow of both the gas and the fluid.

Referring back to FIG. 1, the system 100 further includes hydraulicvalves 128 a, 128 b, 128 c, 128 d . . . 128 n that control thecommunication of the fluid connections of the accumulator 116 and theintensifier 118 with a hydraulic motor 130. The specific number, type,and arrangement of the hydraulic valves 128 and the pneumatic valves 106are collectively referred to as the control valve arrangements. Inaddition, the valves are generally depicted as simple two-way valves(i.e., shut-off valves); however, the valves could essentially be anyconfiguration as needed to control the flow of air and/or fluid in aparticular manner. The hydraulic line between the accumulator 116 andvalves 128 a, 128 b and the hydraulic line between the intensifier 118and valves 128 c, 128 d can include flow sensors 126 that relayinformation to the control system 120.

The motor/pump 130 can be a piston-type assembly having a shaft 131 (orother mechanical coupling) that drives, and is driven by, a combinationelectrical motor and generator assembly 132. The motor/pump 130 couldalso be, for example, an impeller, vane, or gear type assembly. Themotor/generator assembly 132 is interconnected with a power distributionsystem and can be monitored for status and output/input level by thecontrol system 120.

One advantage of the system depicted in FIG. 1, as opposed, for example,to the system of FIGS. 4 and 5, is that it achieves approximately doublethe power output in, for example, a 3000-300 psig range withoutadditional components. Shuffling the hydraulic fluid back and forthbetween the intensifier 118 and the accumulator 116 allows for the samepower output as a system with twice the number of intensifiers andaccumulators while expanding or compressing in the 300-3000 psigpressure range. In addition, this system arrangement can eliminatepotential issues with self-priming for certain the hydraulicmotors/pumps when in the pumping mode (i.e., compression phase).

FIGS. 2A-2Q represent, in a simplified graphical manner, the variousoperational stages of the system 100 during a compression phase, wherethe storage tanks 102 are charged with high pressure air/gas (i.e.,energy is stored). In addition, only one storage tank 102 is shown andsome of the valves and sensors are omitted for clarity. Furthermore, thepressures shown are for reference only and will vary depending on thespecific operating parameters of the system 100.

As shown in FIG. 2A, the system 100 is in a neutral state, where thepneumatic valves 106 and the hydraulic valves 128 are closed. Shut-offvalves 134 are open in every operational stage to maintain theaccumulator 116 and intensifier 118 in communication with the system100. The accumulator fluid chamber 138 is substantially filled, whilethe intensifier fluid chamber 146 is substantially empty. The storagetank 102 is typically at a low pressure (approximately 0 psig) prior tocharging and the hydraulic motor/pump 130 is stationary.

As shown in FIGS. 2B and 2C, as the compression phase begins, pneumaticvalve 106 b is open, thereby allowing fluid communication between theaccumulator air chamber 140 and the intensifier air chamber 144, andhydraulic valves 128 a, 128 d are open, thereby allowing fluidcommunication between the accumulator fluid chamber 138 and theintensifier fluid chamber 146 via the hydraulic motor/pump 130. Themotor/generator 132 (not shown in FIG. 2A; see FIG. 1) begins to drivethe motor/pump 130, and the air pressure between the intensifier 118 andthe accumulator 116 begins to increase, as fluid is driven to theintensifier fluid chamber 146 under pressure. The pressure or mechanicalenergy is transferred to the air chamber 144 via the piston assembly142. This increase of air pressure in the accumulator air chamber 140pressurizes the fluid chamber 138 of the accumulator 116, therebyproviding pressurized fluid to the motor/pump 130 inlet, which caneliminate self-priming concerns.

As shown in FIGS. 2D, 2E, and 2F, the motor/generator 132 continues todrive the motor/pump 130, thereby transferring the hydraulic fluid fromthe accumulator 116 to the intensifier 118, which in turn continues topressurize the air between the accumulator and intensifier air chambers140, 144. FIG. 2F depicts the completion of the first stage of thecompression phase. The pneumatic and hydraulic valves 106, 128 are allclosed. The fluid chamber 144 of the intensifier 118 is substantiallyfilled with fluid at a high pressure (for example, about 3000 psig) andthe accumulator fluid chamber 138 is substantially empty and maintainedat a mid-range pressure (for example, about 250 psig). The pressures inthe accumulator and intensifier air chambers 140, 144 are maintained atthe mid-range pressure.

The beginning of the second stage of the compression phase is shown inFIG. 2G, where hydraulic valves 128 b, 128 c are open and the pneumaticvalves 106 are all closed, thereby putting the intensifier fluid chamber146 at high pressure in communication with the motor/pump 130. Thepressure of any gas remaining in the intensifier air chamber 144 willassist in driving the motor/pump 130. Once the hydraulic pressureequalizes between the accumulator and intensifier fluid chambers 138,146 (as shown in FIG. 2H) the motor/generator will draw electricity todrive the motor/pump 130 and further pressurize the accumulator fluidchamber 138.

As shown in FIGS. 2I and 2J, the motor/pump 130 continues to pressurizethe accumulator fluid chamber 138, which in turn pressurizes theaccumulator air chamber 140. The intensifier fluid chamber 146 is at alow pressure and the intensifier air chamber 144 is at substantiallyatmospheric pressure. Once the intensifier air chamber 144 reachessubstantially atmospheric pressure, pneumatic vent valve 106 c isopened. For a vertical orientation of the intensifier, the weight of theintensifier piston 142 can provide the necessary back-pressure to themotor/pump 130, which would overcome potential self-priming issues forcertain motors/pumps.

As shown in FIG. 2K, the motor/pump 130 continues to pressurize theaccumulator fluid chamber 138 and the accumulator air chamber 140, untilthe accumulator air and fluid chambers are at the high pressure for thesystem 100. The intensifier fluid chamber 146 is at a low pressure andis substantially empty. The intensifier air chamber 144 is atsubstantially atmospheric pressure. FIG. 2K also depicts the change-overin the control valve arrangement when the accumulator air chamber 140reaches the predetermined high pressure for the system 100. Pneumaticvalve 106 a is opened to allow the high pressure gas to enter thestorage tanks 102.

FIG. 2L depicts the end of the second stage of one compression cycle,where all of the hydraulic and the pneumatic valves 128, 106 are closed.The system 100 will now begin another compression cycle, where thesystem 100 shuttles the hydraulic fluid back to the intensifier 118 fromthe accumulator 116.

FIG. 2M depicts the beginning of the next compression cycle. Thepneumatic valves 106 are closed and hydraulic valves 128 a, 128 d areopen. The residual pressure of any gas remaining in the accumulatorfluid chamber 138 drives the motor/pump 130 initially, therebyeliminating the need to draw electricity. As shown in FIG. 2N, anddescribed with respect to FIG. 2G, once the hydraulic pressure equalizesbetween the accumulator and intensifier fluid chambers 138, 146 themotor/generator will draw electricity to drive the motor/pump 130 andfurther pressurize the intensifier fluid chamber 146. During this stage,the accumulator air chamber 140 pressure decreases and the intensifierair chamber 144 pressure increases.

As shown in FIG. 2O, when the gas pressures at the accumulator airchamber 140 and the intensifier air chamber 144 are equal, pneumaticvalve 106 b is opened, thereby putting the accumulator air chamber 140and the intensifier air chamber 144 in fluid communication. As shown inFIGS. 2P and 2Q, the motor/pump 130 continues to transfer fluid from theaccumulator fluid chamber 138 to the intensifier fluid chamber 146 andpressurize the intensifier fluid chamber 146. As described above withrespect to FIGS. 2D-2F, the process continues until substantially all ofthe fluid has been transferred to the intensifier 118 and theintensifier fluid chamber 146 is at the high pressure and theintensifier air chamber 144 is at the mid-range pressure. The system 100continues the process as shown and described in FIGS. 2G-2K to continuestoring high pressure air in the storage tanks 102. The system 100 willperform as many compression cycles (i.e., the shuttling of hydraulicfluid between the accumulator 116 and the intensifier 118) as necessaryto reach a desired pressure of the air in the storage tanks 102 (i.e., afull compression phase).

FIGS. 3A-3M represent, in a simplified graphical manner, the variousoperational stages of the system 100 during an expansion phase, whereenergy (i.e., the stored compressed gas) is recovered. FIGS. 3A-3M usethe same designations, symbols, and exemplary numbers as shown in FIGS.2A-2Q. It should be noted that while the system 100 is described asbeing used to compress the air in the storage tanks 102, alternatively,the tanks 102 could be charged (for example, an initial charge) by aseparate compressor unit.

As shown in FIG. 3A, the system 100 is in a neutral state, where thepneumatic valves 106 and the hydraulic valves 128 are all closed. Thesame as during the compression phase, the shut-off valves 134 are opento maintain the accumulator 116 and intensifier 118 in communicationwith the system 100. The accumulator fluid chamber 138 is substantiallyfilled, while the intensifier fluid chamber 146 is substantially empty.The storage tank 102 is at a high pressure (for example, 3000 psig) andthe hydraulic motor/pump 130 is stationary.

FIG. 3B depicts a first stage of the expansion phase, where pneumaticvalves 106 a, 106 c are open. Open pneumatic valve 106 a connects thehigh pressure storage tanks 102 in fluid communication with theaccumulator air chamber 140, which in turn pressurizes the accumulatorfluid chamber 138. Open pneumatic valve 106 c vents the intensifier airchamber 146 to atmosphere. Hydraulic valves 128 a, 128 d are open toallow fluid to flow from the accumulator fluid chamber 138 to drive themotor/pump 130, which in turn drives the motor/generator 132 (not shownin FIG. 3B), thereby generating electricity. The generated electricitycan be delivered directly to a power grid or stored for later use, forexample, during peak usage times.

As shown in FIG. 3C, once the predetermined volume of pressurized air isadmitted to the accumulator air chamber 140 (for example, 3000 psig),pneumatic valve 106 a is closed to isolate the storage tanks 102 fromthe accumulator air chamber 140. As shown in FIGS. 3C-3F, the highpressure in the accumulator air chamber 140 continues to drive thehydraulic fluid from the accumulator fluid chamber 138 through themotor/pump 130 and to the intensifier fluid chamber 146, therebycontinuing to drive the motor/generator 132 and generate electricity. Asthe hydraulic fluid is transferred from the accumulator 116 to theintensifier 118, the pressure in the accumulator air chamber 140decreases and the air in the intensifier air chamber 144 is ventedthrough pneumatic valve 106C.

FIG. 3G depicts the end of the first stage of the expansion phase. Oncethe accumulator air chamber 140 reaches a second predeterminedmid-pressure (for example, about 300 psig), all of the hydraulic andpneumatic valves 128, 106 are closed. The pressure in the accumulatorfluid chamber 138, the intensifier fluid chamber 146, and theintensifier air chamber 144 are at approximately atmospheric pressure.The pressure in the accumulator air chamber 140 is maintained at thepredetermined mid-pressure.

FIG. 3H depicts the beginning of the second stage of the expansionphase. Pneumatic valve 106 b is opened to allow fluid communicationbetween the accumulator air chamber 140 and the intensifier air chamber144. The predetermined pressure will decrease slightly when the valve106 b is opened and the accumulator air chamber 140 and the intensifierair chamber 144 are connected. Hydraulic valves 128 b, 128 d are opened,thereby allowing the hydraulic fluid stored in the intensifier totransfer to the accumulator fluid chamber 138 through the motor/pump130, which in turn drives the motor/generator 132 and generateselectricity. The air transferred from the accumulator air chamber 140 tothe intensifier air chamber 144 to drive the fluid from the intensifierfluid chamber 146 to the accumulator fluid chamber 138 is at a lowerpressure than the air that drove the fluid from the accumulator fluidchamber 138 to the intensifier fluid chamber 146. The area differentialbetween the air piston 142 a and the fluid piston 142 b (for example,10:1; see FIG. 1B) allows the lower pressure air to transfer the fluidfrom the intensifier fluid chamber 146 at a high pressure.

As shown in FIGS. 3I-3K, the pressure in the intensifier air chamber 144continues to drive the hydraulic fluid from the intensifier fluidchamber 146 through the motor/pump 130 and to the accumulator fluidchamber 138, thereby continuing to drive the motor/generator 132 andgenerate electricity. As the hydraulic fluid is transferred from theintensifier 118 to the accumulator 116, the pressures in the intensifierair chamber 144, the intensifier fluid chamber 146, the accumulator airchamber 140, and the accumulator fluid chamber 138 decrease.

FIG. 3L depicts the end of the second stage of the expansion cycle,where substantially all of the hydraulic fluid has been transferred tothe accumulator 116 and all of the valves 106, 128 are closed. Inaddition, the accumulator air chamber 140, the accumulator fluid chamber138, the intensifier air chamber 144, and the intensifier fluid chamber146 are all at low pressure. In an alternative embodiment, the hydraulicfluid can be shuffled back and forth between two intensifiers forcompressing and expanding in the low pressure (for example, about 0-250psig) range. Using a second intensifier and appropriate valving toutilize the energy stored at the lower pressures can produce additionalelectricity.

FIG. 3M depicts the start of another expansion phase, as described withrespect to FIG. 3B. The system 100 can continue to cycle throughexpansion phases as necessary for the production of electricity, oruntil all of the compressed air in the storage tanks 102 has beenexhausted.

FIG. 4 is a schematic diagram of an energy storage system 300, employingopen-air hydraulic-pneumatic principles according to one embodiment ofthis invention. The system 300 consists of one or more high-pressuregas/air storage tanks 302 a, 302 b, . . . 302 n (the number being highlyvariable to suit a particular application). Each tank 302 a, 302 b isjoined in parallel via a manual valve(s) 304 a, 304 b, . . . 304 nrespectively to a main air line 308. The tanks 302 a, 302 b are eachprovided with a pressure sensor 312 a, 312 b . . . 312 n and atemperature sensor 314 a, 314 b . . . 314 n that can be monitored by asystem controller 350 via appropriate connections (shown generallyherein as arrows indicating “TO CONTROL”). The controller 350, theoperation of which is described in further detail below, can be anyacceptable control device with a human-machine interface. In an oneembodiment, the controller 350 includes a computer 351 (for example aPC-type) that executes a stored control application 353 in the form of acomputer-readable software medium. The control application 353 receivestelemetry from the various sensors and provides appropriate feedback tocontrol valve actuators, motors, and other neededelectromechanical/electronic devices. An appropriate interface can beused to convert data from sensors into a form readable by the computercontroller 351 (such as RS-232 or network-based interconnects).Likewise, the interface converts the computer's control signals into aform usable by valves and other actuators to perform an operation. Theprovision of such interfaces should be clear to those of ordinary skillin the art.

The main air line 308 from the tanks 302 a, 302 b is coupled to a pairof multi-stage (two stages in this example) accumulator/intensifiercircuits (or hydraulic-pneumatic cylinder circuits) (dashed boxes 360,362 in FIG. 4B) via automatically controlled (via controller 350),two-position valves 307 a, 307 b, 307 c and 306 a, 306 b and 306 c.These valves are coupled to respective accumulators 316 and 317 andintensifiers 318 and 319 according to one embodiment of the system.Pneumatic valves 306 a and 307 a are also coupled to a respectiveatmospheric air vent 310 b and 310 a. In particular, valves 306 c and307 c connect along a common air line 390, 391 between the main air line308 and the accumulators 316 and 317, respectively. Pneumatic valves 306b and 307 b connect between the respective accumulators 316 and 317, andintensifiers 318 and 319. Pneumatic valves 306 a, 307 a connect alongthe common lines 390, 391 between the intensifiers 318 and 319, and theatmospheric vents 310 b and 310 a.

The air from the tanks 302, thus, selectively communicates with the airchamber side of each accumulator and intensifier (referenced in thedrawings as air chamber 340 for accumulator 316, air chamber 341 foraccumulator 317, air chamber 344 for intensifier 318, and air chamber345 for intensifier 319). An air temperature sensor 322 and a pressuresensor 324 communicate with each air chamber 341, 344, 345, 322, anddeliver sensor telemetry to the controller 350.

The air chamber 340, 341 of each accumulator 316, 317 is enclosed by amovable piston 336, 337 having an appropriate sealing system usingsealing rings and other components that are known to those of ordinaryskill in the art. The piston 336, 337 moves along the accumulatorhousing in response to pressure differentials between the air chamber340, 341 and an opposing fluid chamber 338, 339, respectively, on theopposite side of the accumulator housing. In this example, hydraulicfluid (or another liquid, such as water) is indicated by a shaded volumein the fluid chamber. Likewise, the air chambers 344, 345 of therespective intensifiers 318, 319 are enclosed by a moving pistonassembly 342, 343. However, the intensifier air piston 342 a, 343 a isconnected by a shaft, rod, or other coupling to a respective fluidpiston, 342 b, 343 b. This fluid piston 342 b, 343 b moves inconjunction with the air piston 342 a, 343 a, but acts directly upon theassociated intensifier fluid chamber 346, 347. Notably, the internaldiameter (and/or volume) of the air chamber (DAI) for the intensifier318, 319 is greater than the diameter of the air chamber (DAA) for theaccumulator 316, 317 in the same circuit 360, 362. In particular, thesurface area of the intensifier pistons 342 a, 343 a is greater than thesurface area of the accumulator pistons 336, 337. The diameter of eachintensifier fluid piston (DFI) is approximately the same as the diameterof each accumulator (DFA). Thus in this manner, a lower air pressureacting upon the intensifier piston generates a similar pressure on theassociated fluid chamber as a higher air pressure acting on theaccumulator piston. In this manner, and as described further below, thesystem allows for at least two stages of pressure to be employed togenerate similar levels of fluid pressure.

In one example, assuming that the initial gas pressure in theaccumulator is at 200 atmospheres (ATM) (3000 psi—high-pressure), with afinal mid-pressure of 20 ATM (300 psi) upon full expansion, and that theinitial gas pressure in the intensifier is then 20 ATM (with a finalpressure of 1.5-2 ATM (25-30 psi)), then the area of the gas piston inthe intensifier would be approximately 10 times the area of the pistonin the accumulator (or 3.16 times the radius). However, the precisevalues for initial high-pressure, mid-pressure and final low-pressureare highly variable, depending in part upon the operating specificationsof the system components, scale of the system and output requirements.Thus, the relative sizing of the accumulators and the intensifiers isvariable to suit a particular application.

Each fluid chamber 338, 339, 346, 347 is interconnected with anappropriate temperature sensor 322 and pressure sensor 324, eachdelivering telemetry to the controller 350. In addition, each fluid lineinterconnecting the fluid chambers can be fitted with a flow sensor 326,which directs data to the controller 350. The pistons 336, 337, 342 and343 can include position sensors 348 that report their present positionto the controller 350. The position of the piston can be used todetermine relative pressure and flow of both gas and fluid. Each fluidconnection from a fluid chamber 338, 339, 346, 347 is connected to apair of parallel, automatically controlled valves. As shown, fluidchamber 338 (accumulator 316) is connected to valve pair 328 c and 328d; fluid chamber 339 (accumulator 317) is connected to valve pair 329 aand 329 b; fluid chamber 346 (intensifier 318) is connected to valvepair 328 a and 328 b; and fluid chamber 347 (intensifier 319) isconnected to valve pair 329 c and 329 d. One valve from each chamber 328b, 328 d, 329 a and 329 c is connected to one connection side 372 of ahydraulic motor/pump 330. This motor/pump 330 can be piston-type (orother suitable type, including vane, impeller, and gear) assembly havinga shaft 331 (or other mechanical coupling) that drives, and is drivenby, a combination electrical motor/generator assembly 332. Themotor/generator assembly 332 is interconnected with a power distributionsystem and can be monitored for status and output/input level by thecontroller 350. The other connection side 374 of the hydraulicmotor/pump 330 is connected to the second valve in each valve pair 328a, 328 c, 329 b and 329 d. By selectively toggling the valves in eachpair, fluid is connected between either side 372, 374 of the hydraulicmotor/pump 330. Alternatively, some or all of the valve pairs can bereplaced with one or more three position, four way valves or othercombinations of valves to suit a particular application.

The number of circuits 360, 362 can be increased as necessary.Additional circuits can be interconnected to the tanks 302 and each side372, 374 of the hydraulic motor/pump 330 in the same manner as thecomponents of the circuits 360, 362. Generally, the number of circuitsshould be even so that one circuit acts as a fluid driver while theother circuit acts as a reservoir for receiving the fluid from thedriving circuit.

An optional accumulator 366 is connected to at least one side (e.g.,inlet side 372) of the hydraulic motor/pump 330. The optionalaccumulator 366 can be, for example, a closed-air-type accumulator witha separate fluid side 368 and precharged air side 370. As will bedescribed below, the accumulator 366 acts as a fluid capacitor to dealwith transients in fluid flow through the motor/pump 330. In anotherembodiment, a second optional accumulator or other low-pressurereservoir 371 is placed in fluid communication with the outlet side 374of the motor/pump 330 and can also include a fluid side 371 and aprecharged air side 369. The foregoing optional accumulators can be usedwith any of the systems described herein.

Having described the general arrangement of one embodiment of anopen-air hydraulic-pneumatic energy storage system 300 in FIG. 4, theexemplary functions of the system 300 during an energy recovery phasewill now be described with reference to FIGS. 5A-5N. For the purposes ofthis operational description, the illustrations of the system 300 inFIGS. 5A-5N have been simplified, omitting the controller 350 andinterconnections with valves, sensors, etc. It should be understood thatthe steps described are under the control and monitoring of thecontroller 350 based upon the rules established by the application 353.

FIG. 5A is a schematic diagram of the energy storage and recovery systemof FIG. 4 showing an initial physical state of the system 300 in whichan accumulator 316 of a first circuit is filled with high-pressure gasfrom the high-pressure gas storage tanks 302. The tanks 302 have beenfilled to full pressure, either by the cycle of the system 300 underpower input to the hydraulic motor/pump 330, or by a separatehigh-pressure air pump 376. This air pump 376 is optional, as the airtanks 302 can be filled by running the recovery cycle in reverse. Thetanks 302 in this embodiment can be filled to a pressure of 200 ATM(3000 psi) or more. The overall, collective volume of the tanks 302 ishighly variable and depends in part upon the amount of energy to bestored.

In FIG. 5A, the recovery of stored energy is initiated by the controller350. To this end, pneumatic valve 307 c is opened allowing a flow ofhigh-pressure air to pass into the air chamber 340 of the accumulator316. Note that where a flow of compressed gas or fluid is depicted, theconnection is indicated as a dashed line. The level of pressure isreported by the sensor 324 in communication with the chamber 340. Thepressure is maintained at the desired level by valve 307 c. Thispressure causes the piston 336 to bias (arrow 800) toward the fluidchamber 338, thereby generating a comparable pressure in theincompressible fluid. The fluid is prevented from moving out of thefluid chamber 338 at this time by valves 329 c and 329 d).

FIG. 5B is a schematic diagram of the energy storage and recovery systemof FIG. 4 showing a physical state of the system 300 following the stateof FIG. 5A, in which valves are opened to allow fluid to flow from theaccumulator 316 of the first circuit to the fluid motor/pump 330 togenerate electricity therefrom. As shown in FIG. 5B, pneumatic valve 307c remains open. When a predetermined pressure is obtained in the airchamber 340, the fluid valve 329 c is opened by the controller, causinga flow of fluid (arrow 801) to the inlet side 372 of the hydraulicmotor/pump 330 (which operates in motor mode during the recovery phase).The motion of the motor 330 drives the electric motor/generator 332 in ageneration mode, providing power to the facility or grid as shown by theterm “POWER OUT.” To absorb the fluid flow (arrow 803) from the outletside 374 of the hydraulic motor/pump 330, fluid valve 328 c is opened tothe fluid chamber 339 by the controller 350 to route fluid to theopposing accumulator 317. To allow the fluid to fill accumulator 317after its energy has been transferred to the motor/pump 330, the airchamber 341 is vented by opening pneumatic vent valves 306 a, 306 b.This allows any air in the chamber 341, to escape to the atmosphere viathe vent 310 b as the piston 337 moves (arrow 805) in response to theentry of fluid.

FIG. 5C is a schematic diagram of the energy storage and recovery systemof FIG. 4 showing a physical state of the system 300 following the stateof FIG. 5B, in which the accumulator 316 of the first circuit directsfluid to the fluid motor/pump 330 while the accumulator 317 of thesecond circuit receives exhausted fluid from the motor/pump 330, as gasin its air chamber 341 is vented to atmosphere. As shown in FIG. 5C, apredetermined amount of gas has been allowed to flow from thehigh-pressure tanks 302 to the accumulator 316 and the controller 350now closes pneumatic valve 307 c. Other valves remain open so that fluidcan continue to be driven by the accumulator 316 through the motor/pump330.

FIG. 5D is a schematic diagram of the energy storage and recovery systemof FIG. 4 showing a physical state of the system 300 following the stateof FIG. 5C, in which the accumulator 316 of the first circuit continuesto direct fluid to the fluid motor/pump 330 while the accumulator 317 ofthe second circuit continues to receive exhausted fluid from themotor/pump 330, as gas in its air chamber 341 is vented to atmosphere.As shown in FIG. 5D, the operation continues, where the accumulatorpiston 336 drives additional fluid (arrow 800) through the motor/pump330 based upon the charge of gas pressure placed in the accumulator airchamber 340 by the tanks 302. The fluid causes the opposingaccumulator's piston 337 to move (arrow 805), displacing air through thevent 310 b.

FIG. 5E is a schematic diagram of the energy storage and recovery systemof FIG. 4 showing a physical state of the system 300 following the stateof FIG. 5D, in which the accumulator 316 of the first circuit has nearlyexhausted the fluid in its fluid chamber 338 and the gas in its airchamber 340 has expanded to nearly mid-pressure from high-pressure. Asshown in FIG. 5E, the charge of gas in the air chamber 340 of theaccumulator 316 has continued to drive fluid (arrows 800, 801) throughthe motor/pump 330 while displacing air via the air vent 310 b. The gashas expanded from high-pressure to mid-pressure during this portion ofthe energy recovery cycle. Consequently, the fluid has ranged from highto mid-pressure. By sizing the accumulators appropriately, the rate ofexpansion can be controlled.

This is part of the significant parameter of heat transfer. For maximumefficiency, the expansion should remain substantially isothermal. Thatis, heat from the environment replaces the heat lost by the expansion.In general, isothermal compression and expansion is critical tomaintaining high round-trip system efficiency, especially if thecompressed gas is stored for long periods. In various embodiments of thesystems described herein, heat transfer can occur through the walls ofthe accumulators and/or intensifiers, or heat-transfer mechanisms canact upon the expanding or compressing gas to absorb or radiate heat fromor to an environmental or other source. The rate of this heat transferis governed by the thermal properties and characteristics of theaccumulators/intensifiers, which can be used to determine a thermal timeconstant. If the compression of the gas in the accumulators/intensifiersoccurs slowly relative to the thermal time constant, then heat generatedby compression of the gas will transfer through theaccumulator/intensifier walls to the surroundings, and the gas willremain at approximately constant temperature. Similarly, if expansion ofthe gas in the accumulators/intensifiers occurs slowly relative to thethermal time constant, then the heat absorbed by the expansion of thegas will transfer from the surroundings through theaccumulator/intensifier walls and to the gas, and the gas will remain atapproximately constant temperature. If the gas remains at a relativelyconstant temperature during both compression and expansion, then theamount of heat energy transferred from the gas to the surroundingsduring compression will equal the amount of heat energy recovered duringexpansion via heat transfer from the surroundings to the gas. Thistransfer is represented by the letter Q and wavy arrows in FIG. 4. Asnoted, a variety of mechanisms can be employed to maintain an isothermalexpansion/compression. In one example, the accumulators can be submergedin a water bath or water/fluid flow can be circulated around theaccumulators and intensifiers. The accumulators can alternatively besurrounded with heating/cooling coils or a flow of warm air can be blownpast the accumulators/intensifiers. However, any technique that allowsfor mass flow transfer of heat to and from the accumulators can beemployed.

FIG. 5F is a schematic diagram of the energy storage and recovery systemof FIG. 4, showing a physical state of the system 300 following thestate of FIG. 5E in which the accumulator 316 of the first circuit hasexhausted the fluid in its fluid chamber 338 and the gas in its airchamber 340 has expanded to mid-pressure from high-pressure, and thevalves have been momentarily closed on both the first circuit and thesecond circuit, while the optional accumulator 366 (shown in FIG. 4)delivers fluid through the motor/pump 330 to maintain operation of theelectric motor/generator 332 between cycles. As shown in FIG. 5F, thepiston 336 of the accumulator 316 has driven all fluid out of the fluidchamber 338 as the gas in the air chamber 340 has fully expanded (tomid-pressure of 20 ATM, per the example). Fluid valves 329 c and 328 care closed by the controller 350. In practice, the opening and closingof valves is carefully timed so that a flow through the motor/pump 330is maintained. However, in an optional implementation, briefinterruptions in fluid pressure can be accommodated by pressurized fluidflow 710 from the optional accumulator (366 in FIG. 4), which isdirected through the motor/pump 330 to the second optional accumulator(367 in FIG. 4) at low-pressure as an exhaust fluid flow 720. In oneembodiment, the exhaust flow can be directed to a simple low-pressurereservoir that is used to refill the first accumulator 366.Alternatively, the exhaust flow can be directed to the second optionalaccumulator (367 in FIG. 4) at low-pressure, which is subsequentlypressurized by excess electricity (driving a compressor) or air pressurefrom the storage tanks 302 when it is filled with fluid. Alternatively,where a larger number of accumulator/intensifier circuits (e.g., threeor more) are employed in parallel in the system 300, their expansioncycles can be staggered so that only one circuit is closed off at atime, allowing a substantially continuous flow from the other circuits.

FIG. 5G is a schematic diagram of the energy storage and recovery systemof FIG. 4 showing a physical state of the system 300 following the stateof FIG. 5F, in which pneumatic valves 307 b, 306 a are opened to allowmid-pressure gas from the air chamber 340 of the first circuit'saccumulator 316 to flow into the air chamber 344 of the first circuit'sintensifier 318, while fluid from the first circuit's intensifier 318 isdirected through the motor/pump 330 and exhausted fluid fills the fluidchamber 347 of second circuit's intensifier 319, whose air chamber 345is vented to atmosphere. As shown in FIG. 5G, pneumatic valve 307 b isopened, while the tank outlet valve 307 c remains closed. Thus, thevolume of the air chamber 340 of accumulator 316 is coupled to the airchamber 344 of the intensifier 318. The accumulator's air pressure hasbeen reduced to a mid-pressure level, well below the initial charge fromthe tanks 302. The air, thus, flows (arrow 810) through valve 307 b tothe air chamber 344 of the intensifier 318. This drives the air piston342 a (arrow 830). Since the area of the air-contacting piston 342 a islarger than that of the piston 336 in the accumulator 316, the lower airpressure still generates a substantially equivalent higher fluidpressure on the smaller-area, coupled fluid piston 342 b of theintensifier 318. The fluid in the fluid chamber 346 thereby flows underpressure through opened fluid valve 329 a and into the inlet side 372 ofthe motor/pump 330. The outlet fluid from the motor pump 330 is directed(arrow 803) through now-opened fluid valve 328 a to the opposingintensifier 319. The fluid enters the fluid chamber 347 of theintensifier 319, biasing (arrow 860) the fluid piston 343 b (andinterconnected gas piston 343 a). Any gas in the air chamber 345 of theintensifier 319 is vented through the now opened vent valve 306 a toatmosphere via the vent 310 b. The mid-level gas pressure in theaccumulator 316 is directed (arrows 810, 820) to the intensifier 318,the piston 342 a of which drives fluid from the chamber 346 using thecoupled, smaller-diameter fluid piston 342 b. This portion of therecovery stage maintains a reasonably high fluid pressure, despite lowergas pressure, thereby ensuring that the motor/pump 330 continues tooperate within a predetermined range of fluid pressures, which isdesirable to maintain optimal operating efficiencies for the givenmotor. Notably, the multi-stage circuits of this embodiment effectivelyrestrict the operating pressure range of the hydraulic fluid deliveredto the motor/pump 330 above a predetermined level despite the wide rangeof pressures within the expanding gas charge provided by thehigh-pressure tank.

FIG. 5H is a schematic diagram of the energy storage and recovery systemof FIG. 4 showing a physical state of the system following the state ofFIG. 5G, in which the intensifier 318 of the first circuit directs fluidto the fluid motor/pump 330 based upon mid-pressure gas from the firstcircuit's accumulator 316 while the intensifier 319 of the secondcircuit receives exhausted fluid from the motor/pump 330, as gas in itsair chamber 345 is vented to atmosphere. As shown in FIG. 5H, the gas inintensifier 318 continues to expand from mid-pressure to low-pressure.Conversely, the size differential between coupled air and fluid pistons342 a and 342 b, respectively, causes the fluid pressure to vary betweenhigh and mid-pressure. In this manner, motor/pump operating efficiencyis maintained.

FIG. 5I is a schematic diagram of the energy storage and recovery systemof FIG. 4 showing a physical state of the system following the state ofFIG. 5H, in which the intensifier 318 of the first circuit has almostexhausted the fluid in its fluid chamber 346 and the gas in its airchamber 344, delivered from the first circuit's accumulator 316, hasexpanded to nearly low-pressure from the mid-pressure. As discussed withrespect to FIG. 5H, the gas in intensifier 318 continues to expand frommid-pressure to low-pressure. Again, the size differential betweencoupled air and fluid pistons 342 a and 342 b, respectively, causes thefluid pressure to vary between high and mid-pressure to maintainmotor/pump operating efficiency.

FIG. 5J is a schematic diagram of the energy storage and recovery systemof FIG. 4 showing a physical state of the system 300 following the stateof FIG. 5I, in which the intensifier 318 of the first circuit hasessentially exhausted the fluid in its fluid chamber 346 and the gas inits air chamber 344, delivered from the first circuit's accumulator 316,has expanded to low-pressure from the mid-pressure. As shown in FIG. 5J,the intensifier's piston 342 reaches full stroke, while the fluid isdriven fully from high to mid-pressure in the fluid chamber 346.Likewise, the opposing intensifier's fluid chamber 347 has filled withfluid from the outlet side 374 of the motor/pump 330.

FIG. 5K is a schematic diagram of the energy storage and recovery systemof FIG. 4 showing a physical state of the system following the state ofFIG. 5J, in which the intensifier 318 of the first circuit has exhaustedthe fluid in its fluid chamber 346 and the gas in its air chamber 344has expanded to low pressure, and the valves have been momentarilyclosed on both the first circuit and the second circuit in preparationof switching-over to an expansion cycle in the second circuit, whoseaccumulator and intensifier fluid chambers 339, 347 are now filled withfluid. At this time, the optional accumulator 366 (not shown in FIG. 5K)can deliver fluid through the motor/pump 330 to maintain operation ofthe motor/generator 332 between cycles. As shown in FIG. 5K, pneumaticvalve 307 b, located between the accumulator 316 and the intensifier 318of the circuit 362, is closed. At this point in the above-describedportion of the recovery stage, the gas charge initiated in FIG. 5A hasbeen fully expanded through two stages with relatively gradual,isothermal expansion characteristics, while the motor/pump 330 hasreceived fluid flow within a desirable operating pressure range. Alongwith pneumatic valve 307 b, the fluid valves 329 a and 328 a (and outletgas valve 307 a) are momentarily closed. The above-described optionalaccumulator 366 (not shown in FIG. 5K), and/or other interconnectedpneumatic/hydraulic accumulator/intensifier circuits, can maintainpredetermined fluid flow through the motor/pump 330 while the valves ofthe subject circuits 360, 362 are momentarily closed. At this time, theoptional accumulators and reservoirs 366, 367, as shown in FIG. 4, canprovide a continuing flow 710 of pressurized fluid through themotor/pump 330, and into the reservoir or low-pressure accumulator(exhaust fluid flow 720). The full range of pressure in the previous gascharge being utilized by the system 300.

FIG. 5L is a schematic diagram of the energy storage and recovery systemof FIG. 4 showing a physical state of the system following the state ofFIG. 5K, in which the accumulator 317 of the second circuit is filledwith high-pressure gas from the high-pressure tanks 302 as part of theswitch-over to the second circuit as an expansion circuit, while thefirst circuit receives exhausted fluid and is vented to atmosphere whilethe optional accumulator 366 delivers fluid through the motor/pump 330to maintain operation of the motor/generator between cycles. As shown inFIG. 5L, the cycle continues with a new charge of high-pressure(slightly lower) gas from the tanks 302 delivered to the opposingaccumulator 317. As shown, pneumatic valve 306 c is now opened by thecontroller 350, allowing a charge of relatively high-pressure gas toflow (arrow 815) into the air chamber 341 of the accumulator 317, whichbuilds a corresponding high-pressure charge in the air chamber 341.

FIG. 5M is a schematic diagram of the energy storage and recovery systemof FIG. 4 showing a physical state of the system following the state ofFIG. 5L, in which valves are opened to allow fluid to flow from theaccumulator 317 of the second circuit to the fluid motor/pump 330 togenerate electricity therefrom, while the first circuit's accumulator316, whose air chamber 340 is vented to atmosphere, receives exhaustedfluid from the motor/pump 330. As shown in FIG. 5M, the pneumatic valve306 c is closed and the fluid valves 328 d and 329 d are opened on thefluid side of the circuits 360, 362, thereby allowing the accumulatorpiston 337 to move (arrow 816) under pressure of the charged air chamber341. This directs fluid under high pressure through the inlet side 372of the motor/pump 330 (arrow 817), and then through the outlet 374. Theexhausted fluid is directed (arrow 818) now to the fluid chamber 338 ofaccumulator 316. Pneumatic valves 307 a and 307 b have been opened,allowing the low-pressure air in the air chamber 340 of the accumulator316 to vent (arrow 819) to atmosphere via vent 310 a. In this manner,the piston 336 of the accumulator 316 can move (arrow 821) withoutresistance to accommodate the fluid from the motor/pump outlet 374.

FIG. 5N is a schematic diagram of the energy storage and recovery systemof FIG. 4 showing a physical state of the system following the state ofFIG. 5M, in which the accumulator 317 of the second circuit 362continues to direct fluid to the fluid motor/pump 330 while theaccumulator 316 of the first circuit continues to receive exhaustedfluid from the motor/pump 330, as gas in its air chamber 340 is ventedto atmosphere, the cycle eventually directing mid-pressure air to thesecond circuit's intensifier 319 to drain the fluid therein. As shown inFIG. 5N, the high-pressure gas charge in the accumulator 317 expandsmore fully within the air chamber 341 (arrow 816). Eventually, thecharge in the air chamber 341 is fully expanded. The mid-pressure chargein the air chamber 341 is then coupled via open pneumatic valve 306 b tothe intensifier 319, which fills the opposing intensifier 318 with spentfluid from the outlet 374. The process repeats until a given amount ofenergy is recovered or the pressure in the tanks 302 drops below apredetermined level.

It should be clear that the system 300, as described with respect toFIGS. 4 and 5A-5N, could be run in reverse to compress gas in the tanks302 by powering the electric generator/motor 332 to drive the motor/pump330 in pump mode. In this case, the above-described process occurs inreverse order, with driven fluid causing compression within both stagesof the air system in turn. That is, air is first compressed to amid-pressure after being drawn into the intensifier from theenvironment. This mid-pressure air is then directed to the air chamberof the accumulator, where fluid then forces it to be compressed to highpressure. The high-pressure air is then forced into the tanks 302. Boththis compression/energy storage stage and the above-describedexpansion/energy recovery stages are discussed with reference to thegeneral system state diagram shown in FIG. 6.

Note that in the above-described systems 100, 300 (i.e., one or morestages, respectively), the compression and expansion cycle is predicatedupon the presence of gas in the storage tanks 302 that is currently at apressure above the mid-pressure level (e.g., above 20 atmospheres). Forsystem 300, for example, when the prevailing pressure in the storagetanks 302 falls below the mid-pressure level (based, for example, uponlevels sensed by tank sensors 312, 314), then the valves can beconfigured by the controller to employ only the intensifier forcompression and expansion. That is, lower gas pressures are accommodatedusing the larger-area gas pistons on the intensifiers, while higherpressures employ the smaller-area gas pistons of the accumulators, 316,317.

Before discussing the state diagram in FIG. 6, it should be noted thatone advantage of the described systems according to this invention isthat, unlike various prior-art systems, this system can be implementedusing generally commercially available components. In the example of asystem having a power output of 10 to 500 kW, for example, high-pressurestorage tanks can be implemented using standard steel or compositecylindrical pressure vessels (e.g. Compressed Natural Gas 5500-psi steelcylinders). The accumulators can be implemented using standard steel orcomposite pressure cylinders with moveable pistons (e.g., afour-inch-inner-diameter piston accumulator). Intensifiers (pressureboosters/multipliers) having characteristics similar to the exemplaryaccumulator can be implemented (e.g., a fourteen-inch booster diameterand four-inch bore diameter single-acting pressure booster availablefrom Parker-Hannifin of Cleveland, Ohio). A fluid motor/pump can be astandard high-efficiency axial piston, radial piston, or gear-basedhydraulic motor/pump, and the associated electrical generator is alsoavailable commercially from a variety of industrial suppliers. Valves,lines, and fittings are commercially available with the specifiedcharacteristics as well.

Having discussed the exemplary sequence of physical steps in variousembodiments of the system, the following is a more general discussion ofoperating states for the system 300 in both the expansion/energyrecovery mode and the compression/energy storage mode. Reference is nowmade to FIG. 6.

In particular, FIG. 6 details a generalized state diagram 600 that canbe employed by the control application 353 to operate the system'svalves and motor/generator based upon the direction of the energy cycle(recovery/expansion or storage/compression) based upon the reportedstates of the various pressure, temperature, piston-position, and/orflow sensors. Base State 1(610) is a state of the system in which allvalves are closed and the system is neither compressing nor expandinggas. A first accumulator and intensifier (e.g., 316, 318) are filledwith the maximum volume of hydraulic fluid and a second accumulator andintensifier (e.g., 317, 319) are filled with the maximum volume of air,which may or may not be at a pressure greater than atmospheric. Thephysical system state corresponding to Base State 1 is shown in FIG. 5A.Conversely, Base State 2 (620) of FIG. 6 is a state of the system inwhich all valves are closed and the system is neither compressing norexpanding gas. The second accumulator and intensifier are filled withthe maximum volume of hydraulic fluid and the first accumulator andintensifier are filled with the maximum volume of air, which may or maynot be at a pressure greater than atmospheric. The physical system statecorresponding to Base State 2 is shown in FIG. 5K.

As shown further in the diagram of FIG. 6, Base State 1 and Base State 2each link to a state termed Single Stage Compression 630. This generalstate represents a series of states of the system in which gas iscompressed to store energy, and which occurs when the pressure in thestorage tanks 302 is less than the mid-pressure level. Gas is admitted(from the environment, for example) into the intensifier (318 or 319,depending upon the current base state), and is then pressurized bydriving hydraulic fluid into that intensifier. When the pressure of thegas in the intensifier reaches the pressure in the storage tanks 302,the gas is admitted into the storage tanks 302. This process repeats forthe other intensifier, and the system returns to the original base state(610 or 620).

The Two Stage Compression 632 shown in FIG. 6 represents a series ofstates of the system in which gas is compressed in two stages to storeenergy, and which occurs when the pressure in the storage tanks 302 isgreater than the mid-pressure level. The first stage of compressionoccurs in an intensifier (318 or 319) in which gas is pressurized tomid-pressure after being admitted at approximately atmospheric (from theenvironment, for example). The second stage of compression occurs inaccumulator (316 or 317) in which gas is compressed to the pressure inthe storage tanks 302 and then allowed to flow into the storage tanks302. Following two stage compression, the system returns to the otherbase state from the current base state, as symbolized on the diagram bythe crossing-over process arrows 634.

The state Single State Expansion 640, as shown in FIG. 6, represents aseries of states of the system in which gas is expanded to recoverstored energy and which occurs when the pressure in the storage tanks302 is less than the mid-pressure level. An amount of gas from storagetanks 302 is allowed to flow directly into an intensifier (318 or 319).This gas then expands in the intensifier, forcing hydraulic fluidthrough the hydraulic motor/pump 330 and into the second intensifier,where the exhausted fluid moves the piston with the gas-side open toatmospheric (or another low-pressure environment). The Single StageExpansion process is then repeated for the second intensifier, afterwhich the system returns to the original base state (610 or 620).

Likewise, the Two Stage Expansion 642, as shown in FIG. 6, represents aseries of states of the system in which gas is expanded in two stages torecover stored energy and which occurs when pressure in the storagetanks is greater than the mid-pressure level. An amount of gas fromstorage tanks 302 is allowed into an accumulator (316 or 317), whereinthe gas expands to mid-pressure, forcing hydraulic fluid through thehydraulic motor/pump 330 and into the second accumulator. The gas isthen allowed into the corresponding intensifier (318 or 319), whereinthe gas expands to near-atmospheric pressure, forcing hydraulic fluidthrough the hydraulic motor/pump 330 and into the second intensifier.The series of states comprising two-stage expansion are shown in theabove-described FIGS. 5A-5N. Following two-stage expansion, the systemreturns to the other base state (610 or 620) as symbolized by thecrossing process arrows 644.

It should be clear that the above-described system for storing andrecovering energy is highly efficient in that it allows for gradualexpansion of gas over a period that helps to maintain isothermalcharacteristics. The system particularly deals with the large expansionand compression of gas between high-pressure to near atmospheric (andthe concomitant thermal transfer) by providing thiscompression/expansion in two or more separate stages that allow for moregradual heat transfer through the system components. Thus little or nooutside energy is required to run the system (heating gas, etc.),rendering the system more environmentally friendly, capable of beingimplemented with commercially available components, and scalable to meeta variety of energy storage/recovery needs. However, it is possible tofurther improve the efficiency of the systems described above byincorporating a heat transfer subsystem as described with respect toFIG. 9.

FIGS. 7A-7F depict the major systems of an alternative system/method ofexpansion/compression cycling an open-air staged hydraulic-pneumaticsystem, where the system 400 includes at least three accumulators 416 a,416 b, 416 c, at least one intensifier 418, and two motors/pumps 430 a,430 b. The compressed gas storage tanks, valves, sensors, etc. are notshown for clarity. FIGS. 7A-7F illustrate the operation of theaccumulators 416, intensifier 418, and the motors/pumps 430 duringvarious stages of expansion (stages 101-106). The system 400 returns tostage 101 after stage 106 is complete.

As shown in the figures, the designations D, F, AI, and F2 refer towhether the accumulator or intensifier is driving (D) or filling (F),with the additional labels for the accumulators where AI refers toaccumulator to intensifier—the accumulator air side attached to anddriving the intensifier air side, and F2 refers to filling at twice therate of the standard filling.

As shown in FIG. 7A the layout consists of three equally sizedhydraulic-pneumatic accumulators 416 a, 416 b, 416 c, one intensifier418 having a hydraulic fluid side 446 with a capacity of about ⅓ of theaccumulator capacity, and two hydraulic motor/pumps 430 a, 430 b.

FIG. 7A represents stage or time instance 101, where accumulator 416 ais being driven with high pressure gas from a pressure vessel. After aspecific amount of compressed gas is admitted (based on the currentvessel pressure), a valve will be closed, disconnecting the pressurevessel and the high-pressure gas will continue to expand in accumulator416 a as shown in FIGS. 7B and 7C (i.e., stages 102 and 103).Accumulator 416 b is empty of hydraulic fluid and its air chamber 440 bis unpressurized and being vented to the atmosphere. The expansion ofthe gas in accumulator 416 a drives the hydraulic fluid out of theaccumulator 416 a, thereby driving the hydraulic motor 430 a, with theoutput of the motor 430 a refilling accumulator 416 b with hydraulicfluid. At the time point shown in 101, accumulator 416 c is at a statewhere gas has already been expanding for two units of time and iscontinuing to drive motor 430 b while filling intensifier 418.Intensifier 418, similar to accumulator 416 b, is empty of hydraulicfluid and its air chamber 440 is unpressurized and being vented to theatmosphere.

Continuing to time instance 102, as shown in FIG. 7B, the air chamber440 a of accumulator 416 a (accumulators as labeled in FIG. 7A)continues to expand, thereby forcing fluid out of the fluid chamber 438a and driving motor/pump 430 a and filling accumulator 416 b.Accumulator 416 c is now empty of hydraulic fluid, but remains atmid-pressure. The air chamber 440 c of accumulator 416 c is nowconnected to the air chamber 440 of intensifier 418. Intensifier 418 isnow full of hydraulic fluid and the mid-pressure gas in accumulator 416c drives the intensifier 418, which provides intensification of themid-pressure gas to high pressure hydraulic fluid. The high-pressurehydraulic fluid drives motor/pump 430 b, with the output of motor/pump430 b also connected to and filling accumulator 416 b throughappropriate valving. Thus, accumulator 416 b is filled at twice thenormal rate when a single expanding hydraulic pneumatic device(accumulator or intensifier) is providing the fluid for filling.

At time instance 103, as shown in FIG. 7C, the system 400 has returnedto a state similar to stage 101, but with different accumulators atequivalent stages. Accumulator 416 b is now full of hydraulic fluid andis being driven with high-pressure gas from a pressure vessel. After aspecific amount of compressed gas is admitted (based on the currentvessel pressure), a valve will be closed, disconnecting the pressurevessel. The high-pressure gas will continue to expand in accumulator 416b as shown in stages 104 and 105. In stage 103, accumulator 416 c isempty of hydraulic fluid and the air chamber 440 c is unpressurized andbeing vented to the atmosphere. The expansion of the gas in accumulator416 b drives the hydraulic fluid out of the accumulator, driving thehydraulic motor motor/pump 430 b, with the output of the motor refillingaccumulator 416 c with hydraulic fluid via appropriate valving. At thetime point shown in 103, accumulator 416 a is at a state where gas hasalready been expanding for two units of time and is continuing to drivemotor/pump 430 a while now filling intensifier 418. Intensifier 418,similar to accumulator 416 c, is again empty of hydraulic fluid and theair chamber 444 is unpressurized and being vented to the atmosphere.

Continuing to time instance 104, as shown in FIG. 7D, the air chamber440 b of accumulator 416 b continues to expand, thereby forcing fluidout of the fluid chamber 438 b and driving motor/pump 430 a and fillingaccumulator 416 c. Accumulator 416 a is now empty of hydraulic fluid,but remains at mid-pressure. The air chamber 440 a of accumulator 416 ais now connected to the air chamber 440 of intensifier 418. Intensifier418 is now full of hydraulic fluid and the mid-pressure gas inaccumulator 416 a drives the intensifier 418, which providesintensification of the mid-pressure gas to high-pressure hydraulicfluid. The high-pressure hydraulic fluid drives motor/pump 430 b, withthe output of motor/pump 430 b also connected to and filling accumulator416 c through appropriate valving. Thus, accumulator 416 c is filled attwice the normal rate (where the normal rate is the rate when a singleexpanding hydraulic pneumatic device, either accumulator or intensifier,is providing the fluid for filling).

At time instance 105, as shown in FIG. 7E, the system 400 has returnedto a state similar to stage 103, but with different accumulators atequivalent stages. Accumulator 416 c is now full of hydraulic fluid andis being driven with high pressure gas from a pressure vessel. After aspecific amount of compressed gas is admitted (based on the currentvessel pressure), a valve will be closed, disconnecting the pressurevessel. The high-pressure gas will continue to expand in accumulator 416c. Accumulator 416 a is empty of hydraulic fluid and the air chamber 440a is unpressurized and being vented to the atmosphere. The expansion ofthe gas in accumulator 416 c drives the hydraulic fluid out of theaccumulator, driving the hydraulic motor motor/pump 430 b, with theoutput of the motor refilling intensifier 418 with hydraulic fluid viaappropriate valving. At the time point shown in 105, accumulator 416 bis at a state where gas has already been expanding for two units of timeand is continuing to drive motor/pump 430 a while filling accumulator416 a with hydraulic fluid via appropriate valving. Intensifier 418,similar to accumulator 416 a, is again empty of hydraulic fluid and theair chamber 444 is unpressurized and being vented to the atmosphere.

Continuing to time instance 106, as shown in FIG. 7F, the air chamber440 c of accumulator 416 c continues to expand, thereby forcing fluidout of the fluid chamber 438 c and driving motor/pump 430 b and fillingaccumulator 416 a. Accumulator 416 b is now empty of hydraulic fluid,but remains at mid-pressure. The air chamber 440 b of accumulator 416 bis now connected to the air chamber 444 of intensifier 418. Intensifier418 is now full of hydraulic fluid and the mid-pressure gas inaccumulator 416 b drives the intensifier 418, which providesintensification of the mid-pressure gas to high-pressure hydraulicfluid. The high-pressure hydraulic fluid drives motor/pump 430 a withthe output of motor/pump 430 a also connected to and filling accumulator416 a through appropriate valving. Thus, accumulator 416 a is filled attwice the normal rate (where the normal rate is the rate when a singleexpanding hydraulic pneumatic device, either accumulator or intensifier,is providing the fluid for filling). Following the states shown in 106,the system returns to the states shown in 101 and the cycle continues.

FIG. 8 is a table illustrating the expansion scheme described above andillustrated in FIGS. 7A-7F for a three-accumulator, one-intensifiersystem. It should be noted that throughout the cycle, twohydraulic-pneumatic devices (two accumulators or one intensifier plusone accumulator) are always expanding and the two motors are alwaysbeing driven, but at different points in the expansion, such that theoverall power remains relatively constant.

FIG. 9 depicts generally a staged hydraulic-pneumatic energy conversionsystem that stores and recovers electrical energy using thermallyconditioned compressed fluids and incorporates various embodiments ofthe invention, for example, those described with respect to FIGS. 1, 4,and 7. As shown in FIG. 9, the system 900 includes five high-pressuregas/air storage tanks 902 a-902 e. Tanks 902 a and 902 b and tanks 902 cand 902 d are joined in parallel via manual valves 904 a, 904 b, 904 c,and 904 d, respectively. Tank 902 e also includes a manual shut-offvalve 904 e. The tanks 902 are joined to a main air line 908 viapneumatic two-way (i.e., shut-off) valves 906 a, 906 b, 906 c. The tankoutput lines include pressure sensors 912 a, 912 b, 912 c. Thelines/tanks 902 could also include temperature sensors. The varioussensors can be monitored by a system controller 960 via appropriateconnections, as described above with respect to FIGS. 1 and 4. The mainair line 908 is coupled to a pair of multi-stage (two-stage, in thisexample) accumulator circuits via automatically controlled pneumaticshut-off valves 907 a, 907 b. These valves 907 a, 907 b are coupled torespective accumulators 916 and 917. The air chambers 940, 941 of theaccumulators 916, 917 are connected, via automatically controlledpneumatic shut-offs 907 c, 907 d, to the air chambers 944, 945 of theintensifiers 918, 919. Pneumatic shut-off valves 907 e, 907 f are alsocoupled to the air line connecting the respective accumulator andintensifier air chambers and to a respective atmospheric air vent 910 a,910 b. This arrangement allows for air from the various tanks 902 to beselectively directed to either accumulator air chamber 944, 945. Inaddition, the various air lines and air chambers can include pressureand temperature sensors 922 924 that deliver sensor telemetry to thecontroller 960.

The system 900 also includes two heat-transfer subsystems 950A, 950B (influid communication with the air chambers 940, 941, 944, 945 of theaccumulators and intensifiers 916-919 and the high-pressure storagetanks 902) that provide improved isothermal expansion and compression ofthe gas. A simplified schematic of one of the heat-transfer subsystems950 is shown in greater detail in FIG. 9A. Each heat-transfer subsystem950 includes a circulation apparatus 952, at least one heat exchanger954, and pneumatic valves 956. One circulation apparatus 952, two heatexchangers 954, and two pneumatic valves 956 are shown in FIGS. 9 and9A, however, the number and type of circulation apparatus 952, heatexchangers 954, and valves 956 can vary to suit a particularapplication. The various components and the operation of theheat-transfer subsystem 950 are described in greater detail hereinbelow.Generally, in one embodiment, the circulation apparatus 952 is apositive-displacement pump capable of operating at pressures up to 3000psi or more and the two heat exchangers 954 are tube-in-shell type (alsoknown as a shell-and-tube type) heat exchangers 954 also capable ofoperating at pressures up to 3000 psi or more. The heat exchangers 954are shown connected in parallel, although they could also be connectedin series. The heat exchangers 954 can have the same or differentheat-transfer areas. For example, where the heat exchangers 954 areconnected in parallel and the first heat exchanger 954A has aheat-transfer area of X and the second heat exchanger 954B has aheat-transfer area of 2×, a control-valve arrangement can be used toselectively direct the gas flow to one or both of the heat exchangers954 to obtain different heat-transfer areas (e.g., X, 2X, or 3X) andthus different thermal efficiencies.

The basic operation of the system 950 is described with respect to FIG.9A. As shown, the system 950 includes the circulation apparatus 952,which can be driven by, for example, an electric motor 953 mechanicallycoupled thereto. Other types of and means for driving the circulationapparatus are contemplated and within the scope of the invention. Forexample, the circulation apparatus 952 could be a combination ofaccumulators, check valves, and an actuator. The circulation apparatus952 is in fluid communication with each of the air chambers 940, 944 viaa three-way, two-position pneumatic valve 956B and draws gas from eitherair chamber 940, 944 depending on the position of the valve 956B. Thecirculation apparatus 952 circulates the gas from the air chamber 940,944 to the heat exchanger 954.

As shown in FIG. 9A, the two heat exchangers 954 are connected inparallel with a series of pneumatic shut-off valves 907G-907J, that canregulate the flow of gas to heat exchanger 954A, heat exchanger 954B, orboth. Also included is a by-pass pneumatic shut-off valve 907K that canbe used to by-pass the heat exchangers 954 (i.e., the heat-transfersubsystem 950 can be operated without circulating gas through eitherheat exchanger). In use, the gas flows through a first side of the heatexchanger 954, while a constant temperature fluid source flows through asecond side of the heat exchanger 954. The fluid source is controlled tomaintain the gas at ambient temperature. For example, as the temperatureof the gas increases during compression, the gas can be directed throughthe heat exchanger 954, while the fluid source (at ambient or coldertemperature) counter flows through the heat exchanger 954 to remove heatfrom the gas. The gas output of the heat exchanger 954 is in fluidcommunication with each of the air chambers 940, 944 via a three-way,two position pneumatic valve 956A that returns the thermally conditionedgas to either air chamber 940, 944, depending on the position of thevalve 956A. The pneumatic valves 956 are used to control from whichhydraulic cylinder the gas is being thermally conditioned.

The selection of the various components will depend on the particularapplication with respect to, for example, fluid flows, heat transferrequirements, and location. In addition, the pneumatic valves can beelectrically, hydraulically, pneumatically, or manually operated. Inaddition, the heat transfer subsystem 950 can include at least onetemperature sensor 922 that, in conjunction with the controller 960,controls the operation of the various valves 907, 956 and thus theoperation of the heat-transfer subsystem 950.

In one exemplary embodiment, the heat transfer subsystem is used with astaged hydraulic-pneumatic energy conversion system as shown anddescribed above, where the two heat exchangers are connected in series.The operation of the heat-transfer subsystem is described with respectto the operation of a 1.5-gallon capacity piston accumulator having a4-inch bore. In one example, the system is capable of producing 1-1.5 kWof power during a 10 second expansion of the gas from 2900 psi to 350psi. Two tube-in-shell heat exchange units (available from SentryEquipment Corp., Oconomowoc, Wis.), one with a heat-transfer area of0.11 m² and the other with a heat exchange area of 0.22 m², are in fluidcommunication with the air chamber of the accumulator. Except for thearrangement of the heat exchangers, the system is similar to that shownin FIG. 9A, and shut-off valves can be used to control the heat-exchangecounter flow, thus providing for no heat exchange, heat exchange with asingle heat exchanger (i.e., with a heat exchange area of 0.11 m² or0.22 m²), or heat exchange with both heat exchangers (i.e., with a heatexchange area of 0.33 m²).

During operation of the systems 900, 950, high-pressure air is drawnfrom the accumulator 916 and circulated through the heat exchangers 954by the circulation apparatus 952. Specifically, once the accumulator 916is filled with hydraulic fluid and the piston is at the top of thecylinder, the gas circulation/heat exchanger sub-circuit and remainingvolume on the air side of the accumulator is filled with 3,000 psi air.The shut-off valves 907G-907J are used to select which, if any, heatexchanger to use. Once this is complete, the circulation apparatus 952is turned on as is the heat exchanger counter-flow. Additionalheat-transfer subsystems are described hereinbelow with respect to FIGS.11-23.

During gas expansion in the accumulator 916, the three-way valves 956are actuated as shown in FIG. 9A and the gas expands. Pressure andtemperature transducers/sensors on the gas side of the accumulator 916are monitored during the expansion, as well as temperaturetransducers/sensors located on the heat transfer subsystem 950. Thethermodynamic efficiency of the gas expansion can be determined when thetotal fluid power energy output is compared to the theoretical energyoutput that could have been obtained by expanding the known volume ofgas in a perfectly isothermal manner.

The overall work output and thermal efficiency can be controlled byadjusting the hydraulic fluid flow rate and the heat-exchanger area.FIG. 10 depicts the relationship between power output, thermalefficiency, and heat-exchanger surface area for this exemplaryembodiment of the systems 900, 950. As shown in FIG. 10, there is atrade-off between power output and efficiency. By increasingheat-exchange area (e.g., by adding heat exchangers to the heat transfersubsystem 950), greater thermal efficiency is achieved over the poweroutput range. For this exemplary embodiment, thermal efficiencies above90% can be achieved when using both heat exchangers 954 for averagepower outputs of ˜1.0 kW. Increasing the gas circulation rate throughthe heat exchangers will also provide additional efficiencies. Based onthe foregoing, the selection and sizing of the components can beaccomplished to optimize system design, by balancing cost and size withpower output and efficiency.

The basic operation and arrangement of the system 900 is substantiallysimilar to that of systems 100 and 300; however, there are differencesin the arrangement of the hydraulic valves, as described herein.Referring back to FIG. 9 for the remaining description of the basicstaged hydraulic-pneumatic energy conversion system 900, the air chamber940, 941 of each accumulator 916, 917 is partially bounded by a moveablepiston 936, 937 having an appropriate sealing system using sealing ringsand other components that are known to those of ordinary skill in theart. The piston 936, 937 moves along the accumulator housing in responseto pressure differentials between the air chamber 940, 941 and anopposing fluid chamber 938, 939, respectively, on the opposite side ofthe accumulator housing. Likewise, the air chambers 944, 945 of therespective intensifiers 918, 919 are also partially bounded by amoveable piston assembly 942, 943. However, the piston assembly 942, 943includes an air piston connected by a shaft, rod, or other coupling to arespective fluid piston that moves in conjunction. The differencesbetween the piston diameters allow a lower air pressure acting upon theair piston to generate a similar pressure on the associated fluidchamber as the higher air pressure acting on the accumulator piston. Inthis manner, and as previously described, the system allows for at leasttwo stages of pressure to be employed to generate similar levels offluid pressure.

The accumulator fluid chambers 938, 939 are interconnected to ahydraulic motor/pump arrangement 930 via a hydraulic valve 928 a. Thehydraulic motor/pump arrangement 930 includes a first port 931 and asecond port 933. The arrangement 930 also includes several optionalvalves, including a normally open shut-off valve 925, a pressure reliefvalve 927, and three check valves 929 that can further control theoperation of the motor/pump arrangement 930. For example, check valves929 a, 929 b may direct fluid flow from the motor/pump's leak port tothe port 931, 933 at a lower pressure. In addition, valves 925, 929 cprevent the motor/pump from coming to a hard stop during an expansioncycle.

The hydraulic valve 928 a is shown as a 3-position, 4-way directionalvalve that is electrically actuated and spring returned to a centerclosed position, where no flow through the valve 928 a is possible inthe unactuated state. The directional valve 928 a controls the fluidflow from the accumulator fluid chambers 938, 939 to either the firstport 931 or the second port 933 of the motor/pump arrangement 930. Thisarrangement allows fluid from either accumulator fluid chamber 938, 939to drive the motor/pump 930 clockwise or counter-clockwise via a singlevalve.

The intensifier fluid chambers 946, 947 are also interconnected to thehydraulic motor/pump arrangement 930 via a hydraulic valve 928 b. Thehydraulic valve 928 b is also a 3-position, 4-way directional valve thatis electrically actuated and spring returned to a center closedposition, where no flow through the valve 928 b is possible in theunactuated state. The directional valve 928 b controls the fluid flowfrom the intensifier fluid chambers 946, 947 to either the first port931 or the second port 933 of the motor/pump arrangement 930. Thisarrangement allows fluid from either intensifier fluid chamber 946, 947to drive the motor/pump 930 clockwise or counter-clockwise via a singlevalve.

The motor/pump 930 can be coupled to an electrical generator/motor andthat drives, and is driven by the motor/pump 930. As discussed withrespect to the previously described embodiments, the generator/motorassembly can be interconnected with a power distribution system and canbe monitored for status and output/input level by the controller 960.

In addition, the fluid lines and fluid chambers can include pressure,temperature, or flow sensors and/or indicators 922, 924 (not all ofwhich are explicitly labeled in FIG. 9) that deliver sensor telemetry tothe controller 960 and/or provide visual indication of an operationalstate. In addition, the pistons 936, 937, 942, 943 can include positionsensors 948 that report their present position to the controller 960.The position of the piston can be used to determine relative pressureand flow of both gas and fluid.

FIG. 11 is an illustrative embodiment of an isothermal-expansionhydraulic/pneumatic system in accordance with one simplified embodimentof the invention. The system consists of a cylinder 1101 containing agas chamber or “pneumatic side” 1102 and a fluid chamber or “hydraulicside” 1104 separated by a movable (double arrow 1140) piston 1103 orother force/pressure-transmitting barrier that isolates the gas from thefluid. The cylinder 1101 can be a conventional, commercially availablecomponent, modified to receive additional ports as described below. Aswill also be described in further detail below, any of the embodimentsdescribed herein can be implemented as an accumulator or intensifier inthe hydraulic and pneumatic circuits of the energy storage and recoverysystems described above (e.g., accumulator 316, intensifier 318). Thecylinder 1101 includes a primary gas port 1105, which can be closed viavalve 1106 and that connects with a pneumatic circuit, or any otherpneumatic source/storage system. The cylinder 1101 further includes aprimary fluid port 1107 that can be closed by valve 1108. This fluidport connects with a source of fluid in the hydraulic circuit of theabove-described storage system, or any other fluid reservoir.

With reference now to the heat-transfer subsystem 1150, the cylinder1101 has one or more gas circulation output ports 1110 that areconnected via piping 1111 to the gas circulator 1152. Note, as usedherein the term “pipe,” “piping” and the like shall refer to one or moreconduits that are rated to carry gas or other fluids between two points.Thus, the singular term should be taken to include a plurality ofparallel conduits where appropriate. The gas circulator 1152 can be aconventional or customized low-head pneumatic pump, fan, or any otherdevice for circulating gas. The gas circulator 1152 should be sealed andrated for operation at the pressures contemplated within the gas chamber1102. Thus, the gas circulator 1152 creates a predetermined flow (arrow1130) of gas up the piping 1111 and therethrough. The gas circulator1152 can be powered by electricity from a power source or by anotherdrive mechanism, such as a fluid motor. The mass-flow speed and on/offfunctions of the circulator 1152 can be controlled by a controller 1160acting on the power source for the circulator 1152. The controller 1160can be a software and/or hardware-based system that carries out theheat-exchange procedures described herein. The output of the gascirculator 1152 is connected via a pipe 1114 to the gas input 1115 of aheat exchanger 1154.

The heat exchanger 1154 of the illustrative embodiment can be anyacceptable design that allows energy to be efficiently transferred toand from a high-pressure gas flow contained within a pressure conduit toanother mass flow (fluid). The rate of heat exchange is based, in parton the relative flow rates of the gas and fluid, the exchange surfacearea between the gas and fluid and the thermal conductivity of theinterface therebetween. In particular, the gas flow is heated in theheat exchanger 1154 by the fluid counter-flow 1117 (arrows 1126), whichenters the fluid input 1118 of heat exchanger 1154 at ambienttemperature and exits the heat exchanger 1154 at the fluid exit 1119equal or approximately equal in temperature to the gas in piping 1114.The gas flow at gas exit 1120 of heat exchanger 1154 is at ambient orapproximately ambient temperature, and returns via piping 1121 throughone or more gas circulation input ports 1122 to gas chamber 1102. By“ambient” it is meant the temperature of the surrounding environment, oranother desired temperature at which efficient performance of the systemcan be achieved. The ambient-temperature gas reentering the cylinder'sgas chamber 1102 at the circulation input ports 1122 mixes with the gasin the gas chamber 1102, thereby bringing the temperature of the fluidin the gas chamber 1102 closer to ambient temperature.

The controller 1160 manages the rate of heat exchange based, forexample, on the prevailing temperature (T) of the gas contained withinthe gas chamber 1102 using a temperature sensor 1113B of conventionaldesign that thermally communicates with the gas within the chamber 1102.The sensor 1113B can be placed at any location along the cylinderincluding a location that is at, or adjacent to, the heat exchanger gasinput port 1110. The controller 1160 reads the value T from the cylindersensor and compares it to an ambient temperature value (TA) derived froma sensor 1113C located somewhere within the system environment. When Tis greater than TA, the heat-transfer subsystem 1150 is directed to movegas (by powering the circulator 1152) therethrough at a rate that can bepartly dependent upon the temperature differential (so that the exchangedoes not overshoot or undershoot the desired setting). Additionalsensors can be located at various locations within the heat exchangesubsystem to provide additional telemetry that can be used by a morecomplex control algorithm. For example, the output gas temperature (TO)from the heat exchanger can measured by a sensor 1113A that is placedupstream of the outlet port 1122.

The fluid circuit of the heat exchanger 1150 can be filled with water, acoolant mixture, and/or any acceptable heat-transfer medium. Inalternative embodiments, a gas, such as air or refrigerant, can be usedas the heat-transfer medium. In general, the fluid is routed by conduitsto a large reservoir of such fluid in a closed or open loop. One exampleof an open loop is a well or body of water from which ambient water isdrawn and the exhaust water is delivered to a different location, forexample, downstream in a river. In a closed loop embodiment, a coolingtower can cycle the water through the air for return to the heatexchanger. Likewise, water can pass through a submerged or buried coilof continuous piping where a counter heat-exchange occurs to return thefluid flow to ambient before it returns to the heat exchanger foranother cycle.

It should also be clear that the isothermal operation of the inventionworks in two directions thermodynamically. While the gas is warmed toambient by the fluid during expansion, the gas can also be cooled toambient by the heat exchanger during compression, as significantinternal heat can build up via compression. The heat exchangercomponents should be rated, thus, to handle the temperature rangeexpected to be encountered for entering gas and exiting fluid. Moreover,since the heat exchanger is external of the hydraulic/pneumaticcylinder, it can be located anywhere that is convenient and can be sizedas needed to deliver a high rate of heat exchange. In addition it can beattached to the cylinder with straightforward taps or ports that arereadily installed on the base end of an existing, commercially availablehydraulic/pneumatic cylinder.

Reference is now made to FIG. 12, which details a second illustrativeembodiment of an isothermal-expansion hydraulic/pneumatic system inaccordance with one simplified embodiment of the invention. In thisembodiment, the heat-exchange subsystem 1250 is similar or identical tothe heat-exchange subsystems 950, 1150 described above. Thus, where likecomponents are employed, they are given like reference numbers herein.The illustrative system in this embodiment comprises an “intensifier”consisting of a cylinder assembly 1201 containing a gas chamber 1202 anda fluid chamber 1204 separated by a piston assembly 1203. The pistonassembly 1203 in this arrangement consists of a larger diameter/areapneumatic piston member 1210 tied by a shaft 1212 to a smallerdiameter/area hydraulic piston 1214. The corresponding gas chamber 1202is thus larger in cross section than the fluid chamber 1204 and isseparated by a moveable (double arrow 420) piston assembly 1203. Therelative dimensions of the piston assembly 1203 result in a differentialpressure response on each side of the cylinder 1201. That is, thepressure in the gas chamber 1202 can be lower by some predeterminedfraction relative to the pressure in the fluid chamber as a function ofeach piston members' 1210, 1214 relative surface area.

As previously discussed, any of the embodiments described herein can beimplemented as an accumulator or intensifier in the hydraulic andpneumatic circuits of the energy storage and recovery systems describedabove. For example, intensifier cylinder 1201 can be used as a stagealong with the cylinder 1101 of FIG. 11, in the previously describedsystems. To interface with those systems or another application, thecylinder 1201 can include a primary gas port 1205 that can be closed viavalve 1206 and a primary fluid port 1207 that can be closed by valve1208.

With reference now to the heat-exchange subsystem 1250, the intensifiercylinder 1201 also has one or more gas circulation output ports 1210that are connected via piping 1211 to a gas circulator 1252. Again, thegas circulator 1252 can be a conventional or customized low-headpneumatic pump, fan, or any other device for circulating gas. The gascirculator 1252 should be sealed and rated for operation at thepressures contemplated within the gas chamber 1202. Thus, the gascirculator 1252 creates a predetermined flow (arrow 1230) of gas up thepiping 1211 and therethrough. The gas circulator 1252 can be powered byelectricity from a power source or by another drive mechanism, such as afluid motor. The mass-flow speed and on/off functions of the circulator1252 can be controlled by a controller 1260 acting on the power sourcefor the circulator 1252. The controller 1260 can be a software and/orhardware-based system that carries out the heat-exchange proceduresdescribed herein. The output of the gas circulator 1252 is connected viaa pipe 1214 to the gas input 1215 of a heat exchanger 1254.

Again, the gas flow is heated in the heat exchanger 1254 by the fluidcounter-flow 1217 (arrows 1226), which enters the fluid input 1218 ofheat exchanger 1254 at ambient temperature and exits the heat exchanger1254 at the fluid exit 1219 equal or approximately equal in temperatureto the gas in piping 1214. The gas flow at gas exit 1220 of heatexchanger 1254 is at approximately ambient temperature, and returns viapiping 1221 through one or more gas circulation input ports 1222 to gaschamber 1202. By “ambient” is meant the temperature of the surroundingenvironment, or another desired temperature at which efficientperformance of the system can be achieved. The ambient-temperature gasreentering the cylinder's gas chamber 1202 at the circulation inputports 1222 mixes with the gas in the gas chamber 1202, thereby bringingthe temperature of the fluid in gas chamber 1202 closer to ambienttemperature. Again, the heat-transfer subsystem 1250 when used inconjunction with the intensifier of FIG. 12 may be particularly sizedand arranged to accommodate the performance of the intensifier's gaschamber 1202, which may differ thermodynamically from that of thecylinder's gas chamber 1102 in the embodiment shown in FIG. 11.Nevertheless, it is contemplated that the basic structure and functionof heat exchangers in both embodiments is generally similar. Likewise,the controller 1260 can be adapted to deal with the performance curve ofthe intensifier cylinder. As such, the temperature readings of thechamber sensor 1213B, ambient sensor 1213C, and exchanger output sensor1213A are similar to those described with respect to sensors 1113 inFIG. 11. A variety of alternate sensor placements are expresslycontemplated in this embodiment.

Reference is now made to FIG. 13, which shows the cylinder 1101 and heattransfer subsystem 1150 shown and described in FIG. 11, in combinationwith a potential circuit 1370. This embodiment illustrates the abilityof the cylinder 1101 to perform work. The above-described intensifier1201 can likewise be arranged to perform work in the manner shown inFIG. 13. In summary, as the pressurized gas in the gas chamber 1102expands, the gas performs work on piston assembly 1103 as shown (or onpiston assembly 1203 in the embodiment of FIG. 12), which performs workon fluid in fluid chamber 1104 (or fluid chamber 1204), thereby forcingfluid out of fluid chamber 1104 (1204). Fluid forced out of fluidchamber 1104 (1204) flows via piping 1371 to a hydraulic motor 1372 ofconventional design, causing the hydraulic motor 1372 to drive a shaft1373. The shaft 1373 drives an electric motor/generator 1374, generatingelectricity. The fluid entering the hydraulic the motor 1372 exits themotor and flows into fluid receptacle 1375. In such a manner, energyreleased by the expansion of gas in gas chamber 1102 (1202) is convertedto electric energy. The gas may be sourced from an array ofhigh-pressure storage tanks as described above. The heat-exchangesubsystem may maintain ambient temperature in the gas chamber 1102(1202) in the manner described above during the expansion process.

In a similar manner, electric energy can be used to compress gas,thereby storing energy. Electric energy supplied to the electricmotor/generator 1374 drives the shaft 1373 that, in turn, drives thehydraulic motor 1372 in reverse. This action forces fluid from fluidreceptacle 1375 into piping 1371 and further into fluid chamber 1104(1204) of the cylinder 1101. As fluid enters fluid chamber 1104 (1204),it performs work on the piston assembly 1103, which thereby performswork on the gas in the gas chamber 1102 (1202), i.e., compresses thegas. The heat-exchange subsystem 1150 can be used to remove heatproduced by the compression and maintain the temperature at ambient ornear-ambient by proper reading by the controller 1160 (1260) of thesensors 1113 (1213), and throttling of the circulator 1152 (1252).

Reference is now made to FIGS. 14A, 14B, and 14C, which respectivelyshow the ability to perform work when the cylinder or intensifierexpands gas adiabatically, isothermally, or nearly isothermally. Withreference first to FIG. 14A, if the gas in a gas chamber expands from aninitial pressure 502 and an initial volume 504 quickly enough that thereis virtually no heat input to the gas, then the gas expandsadiabatically, following adiabatic curve 506 a, until the gas reachesatmospheric pressure 508 and adiabatic final volume 510 a. The workperformed by this adiabatic expansion is shaded area 512 a. Clearly, asmall portion of the curve becomes shaded, indicating a smaller amountof work performed and an inefficient transfer of energy.

Conversely, as shown in FIG. 14B, if the gas in the gas chamber expandsfrom the initial pressure 502 and the initial volume 504 slowly enoughthat there is perfect heat transfer into the gas, then the gas willremain at a constant temperature and will expand isothermally, followingisothermal curve 506 b until the gas reaches atmospheric pressure 508and isothermal final volume 510 b. The work performed by this isothermalexpansion is shaded area 512 b. The work 512 b achieved by isothermalexpansion 506 b is significantly greater than the work 512 a achieved byadiabatic expansion 506 a. Achieving perfect isothermal expansion may bedifficult in all circumstances, as the amount of time requiredapproaches infinity. Actual gas expansion resides between isothermal andadiabatic.

The heat transfer subsystems 950, 1150, 1250 in accordance with theinvention contemplate the creation of at least an approximate ornear-perfect isothermal expansion as indicated by the graph of FIG. 14C.Gas in the gas chamber expands from the initial pressure 502 and theinitial volume 504 following actual expansion curve 506 c, until the gasreaches atmospheric pressure 508 and actual final volume 510 c. Theactual work performed by this expansion is shaded area 512 c. If actualexpansion 506 c is near-isothermal, then the actual work 512 c performedwill be approximately equal to the isothermal work 512 b (when comparingthe area in FIG. 14B). The ratio of the actual work 512 c divided by theperfect isothermal work 512 b is the thermal efficiency of the expansionas plotted on the y-axis of FIG. 10.

The power output of the system is equal to the work done by theexpansion of the gas divided by the time it takes to expand the gas. Toincrease the power output, the expansion time needs to be decreased. Asthe expansion time decreases, the heat transfer to the gas willdecrease, the expansion will be more adiabatic, and the actual workoutput will be less, i.e., closer to the adiabatic work output. Inembodiments of the invention described herein, heat transfer to the gasis increased by increasing the surface area over which heat transfer canoccur in a circuit external to, but in fluid communication with, theprimary air chamber, as well as the rate at which that gas is passedover the heat exchange surface area. This arrangement increases the heattransfer to/from the gas and allows the work output to remain constantand approximately equal to the isothermal work output even as theexpansion time decreases, resulting in a greater power output. Moreover,embodiments of the systems and methods described herein enable the useof commercially available components that, because they are locatedexternally, can be sized appropriately and positioned anywhere that isconvenient within the footprint of the system.

It should be clear to those of ordinary skill that the design of theheat exchanger and flow rate of the pump can be based upon empiricalcalculations of the amount of heat absorbed or generated by eachcylinder during a given expansion or compression cycle so that theappropriate exchange surface area and fluid flow is provided to satisfythe heat transfer demands. Likewise, an appropriately sized heatexchanger can be derived, at least in part, through experimentaltechniques, after measuring the needed heat transfer and providing theappropriate surface area and flow rate.

FIG. 15 is a schematic diagram of a system and method for expedited heattransfer to gas expanding (or being compressed) in an open-air stagedhydraulic-pneumatic system. The systems and methods previously describedcan be modified to improve heat transfer by replacing the singlehydraulic-pneumatic accumulators with a series of long narrowpiston-based accumulators 1517. The air and hydraulic fluid sides ofthese piston-based accumulators are tied together at the ends (e.g., bya machined metal block 1521 held in place with tie rods) to mimic asingle accumulator with one air input/output 1532 and one hydraulicfluid input/output 1532. The bundle of piston-based accumulators 1517are enclosed in a shell 1523, which can contain a fluid (e.g., water)that can be circulated past the bundle of accumulators 1517 (e.g.,similar to a tube-in-shell heat exchanger) during air expansion orcompression to expedite heat transfer. This entire bundle-and-shellarrangement forms the modified accumulator 1516. The fluid input 1527and fluid output 1529 from the shell 1523 can run to an environmentalheat exchanger or to a source of process heat, cold water, or otherexternal heat exchange medium.

Also shown in FIG. 15 is a modified intensifier 1518. The function ofthe intensifier is identical to those previously described; however,heat exchange between the air expanding (or being compressed) isexpedited by the addition of a bundle of long, narrow, low-pressurepiston-based accumulators 1519. This bundle of accumulators 1519 allowsfor expedited heat transfer to the air. The hydraulic fluid from thebundle of piston-based accumulators 1519 is low pressure (equal to thepressure of the expanding air). The pressure is intensified in ahydraulic-fluid to hydraulic-fluid intensifier (booster) 1520, thusmimicking the role of the air-to-hydraulic fluid intensifiers describedabove, except for the increased surface area for heat exchange duringexpansion/compression. Similar to modified accumulator 1516, this bundleof piston-based accumulators 1519 is enclosed in a shell 1525 and, alongwith the booster, mimics a single intensifier with one air input/output1531 and one hydraulic fluid input/output 1533. The shell 1525 cancontain a fluid (e.g., water) that can be circulated past the bundle ofaccumulators 1519 during air expansion or compression to expedite heattransfer. The fluid input 1526 and fluid output 1528 from the shell 1525can run to an environmental heat exchanger or to a source of processheat, cold water, or other external heat exchange medium.

FIG. 16 is a schematic diagram of an alternative system and method forexpedited heat transfer of gas expanding (or being compressed) in anopen-air staged hydraulic-pneumatic system. In this setup, the systemdescribed in FIG. 15 is modified to reduce costs and potential issueswith piston friction as the diameter of the long narrow piston-basedaccumulators is further reduced. In this embodiment, a series of longnarrow fluid-filled (e.g. water) tubes (e.g. piston-less accumulators)1617 is used in place of the many piston-based accumulators 1517 in FIG.15. In this way, cost is substantially reduced, as the tubes no longerneed to be honed to a high-precision diameter and no longer need to bestraight for piston travel. Similar to those described in FIG. 15, thesebundles of fluid-filled tubes 1617 are tied together at the ends tomimic a single tube (piston-less accumulator) with one air input/output1630 and one hydraulic fluid input/output 1632. The bundle of tubes 1617is enclosed in a shell 1623, which can contain a fluid (e.g., water) atlow pressure, which can be circulated past the bundle of tubes 1617during air expansion or compression to expedite heat transfer. Thisentire bundle-and-shell arrangement forms the modified accumulator 1616.The input 1627 and output 1629 from the shell 1623 can run to anenvironmental heat exchanger or to a source of process heat, cold water,or other external heat-exchange medium. In addition, a fluid- (e.g.,water) to-hydraulic-fluid piston-based accumulator 1622 can be used totransmit the pressure from the fluid (water) in accumulator 1616 to ahydraulic fluid, eliminating worries about air in the hydraulic fluid.

Also shown in FIG. 16 is a modified intensifier 1618. The function ofthe intensifier 1618 is identical to that of those previously described;however, heat exchange between the air expanding (or being compressed)is expedited by the addition of a bundle of the long narrow low-pressuretubes (piston-less accumulators) 1619. This bundle of accumulators 1619allows for expedited heat transfer to the air. The hydraulic fluid fromthe bundle of piston-based accumulators 1619 is low-pressure (equal tothe pressure of the expanding air). The pressure is intensified in ahydraulic-fluid to hydraulic-fluid intensifier (booster) 1620, thusmimicking the role of the air-to-hydraulic fluid intensifiers describedabove, except for the increased surface area for heat exchange duringexpansion/compression and with reduced cost and friction as comparedwith the intensifier 1518 described in FIG. 15. Similar to modifiedaccumulator 1616, this bundle of piston-based accumulators 1619 isenclosed in a shell 1625 and, along with the booster 1620, mimics asingle intensifier with one air input/output 1631 and one hydraulicfluid input/output 1633. The shell 1625 can contain a fluid (e.g.,water) that can be circulated past the bundle of accumulators 1619during air expansion or compression to expedite heat transfer. The fluidinput 1626 and fluid output 1628 from the shell 1625 can run to anenvironmental heat exchanger or to a source of process heat, cold water,or other external heat exchange medium.

FIG. 17 is a schematic diagram of another alternative system and methodfor expedited heat transfer to gas expanding (or being compressed) in anopen-air staged hydraulic-pneumatic system. In this setup, the system ofFIG. 11 is modified to eliminate dead air space and potentially improveheat transfer by using a liquid-to-liquid heat exchanger. As shown inFIG. 11, an air circulator 1152 is connected to the air space ofpneumatic-hydraulic cylinder 1101. One possible drawback of the aircirculator system is that some “dead air space” is present and canreduce the energy efficiency by having some air expansion without usefulwork being extracted.

Similar to the cylinder 1101 shown in FIG. 11, the cylinder 1701includes a primary gas port 1705, which can be closed via a valve andconnected with a pneumatic circuit, or any other pneumaticsource/storage system. The cylinder 1701 further includes a primaryfluid port 1707 that can be closed by a valve. This fluid port connectswith a source of fluid in the hydraulic circuit of the above-describedstorage systems, or any other fluid reservoir.

As shown in FIG. 17, a water circulator 1752 is attached to thepneumatic side 1702 of the hydraulic-pneumatic cylinder (accumulator orintensifier) 1701. Sufficient fluid (e.g., water) is added to thepneumatic side 1702, such that no dead space is present—e.g., theheat-transfer subsystem 1750 (i.e., circulator 1752 and heat exchanger1754) are filled with fluid—when the piston 1701 is fully to the top(e.g., hydraulic side 1704 is filled with hydraulic fluid).Additionally, enough extra liquid is present in the pneumatic side 1702such that liquid can be drawn out of the bottom of the cylinder 1701when the piston is fully at the bottom (e.g., hydraulic side 1704 isempty of hydraulic fluid). As the gas is expanded (or being compressed)in the cylinder 1701, the liquid is circulated by liquid circulator 1752through a liquid-to-liquid heat exchanger 1754, which may be ashell-and-tube type with the input 1722 and output 1724 from the shellrunning to an environmental heat exchanger or to a source of processheat, cold water, or other external heat exchange medium. The liquidthat is circulated by circulator 1752 (at a pressure similar to theexpanding gas in the pneumatic side 1702) is sprayed back into thepneumatic side 1702 after passing through the heat exchanger 1754, thusincreasing the heat exchange between the liquid and the expanding air.Overall, this method allows for dead-space volume to be filled with anincompressible liquid; thus, the heat-exchanger volume can be large andit can be located anywhere that is convenient. By removing all heatexchangers from the cylinders themselves, the overall efficiency of theenergy storage system can be increased. Likewise, as liquid-to-liquidheat exchangers tend to more efficient than air-to-liquid heatexchangers, heat transfer may be improved. It should be noted that inthis particular arrangement, the hydraulic/pneumatic cylinder 1701 wouldbe oriented horizontally, so that liquid pools on the lengthwise base ofthe cylinder 1701 to be continually drawn into circulator 1752.

FIG. 18 is a schematic diagram of another alternative system and methodfor expedited heat transfer to gas expanding (or being compressed) in anopen-air staged hydraulic-pneumatic system. In this setup, the system ofFIG. 11 is again modified to eliminate dead air space and potentiallyimprove heat transfer by using a liquid-to-liquid heat exchanger in asimilar manner as described with respect to FIG. 17. Also, the cylinder1801 can include a primary gas port 1805, which can be closed via avalve and connected with a pneumatic circuit, or any other pneumaticsource/storage system, and a primary fluid port 1807 that can be closedby a valve and connected with a source of fluid in the hydraulic circuitof the above-described storage systems, or any other fluid reservoir.

The heat-exchange subsystem shown in FIG. 18, however, includes a hollowrod 1803 attached to the piston of the hydraulic-pneumatic cylinder(accumulator or intensifier) 1801 such that liquid can be sprayedthroughout the entire volume of the pneumatic side 1802 of the cylinder1801, thereby increasing the heat exchange between the liquid and theexpanding air over FIG. 17, where the liquid is only sprayed from theend cap. Rod 1803 is attached to the pneumatic side 1802 of the cylinder1801 and runs through a seal 1811, such that the liquid in a pressurizedreservoir or vessel 1813 (e.g., a metal tube with an end cap attached tothe cylinder 1801) can be pumped to a slightly higher pressure than thegas in the cylinder 1801.

As the gas is expanding (or being compressed) in the cylinder 1801, theliquid is circulated by circulator 1852 through a liquid-to-liquid heatexchanger 1854, which may be a shell-and-tube type with the input 1822and output 1824 from the shell running to an environmental heatexchanger or to a source of process heat, cold water, or other externalheat exchange medium. Alternatively, a liquid-to-air heat exchangercould be used. The liquid is circulated by circulator 1852 through aheat exchanger 1854 and then sprayed back into the pneumatic side 1802of the cylinder 1801 through the rod 1803, which has holes drilled alongits length. Overall, this set-up allows for dead-space volume to befilled with an incompressible liquid; thus, the heat-exchanger volumecan be large and it can be located anywhere. Likewise, as liquid toliquid heat exchangers tend to more efficient than air to liquid heatexchangers, heat transfer may be improved. By adding the spray rod 1803,the liquid can be sprayed throughout the entire gas volume increasingheat transfer over the set-up shown in FIG. 17.

FIG. 19 is a schematic diagram of another alternative system and methodfor expedited heat transfer to gas expanding (or being compressed) in anopen-air staged hydraulic-pneumatic system. In this setup, the system isarranged to eliminate dead air space and potentially improve heattransfer by using a liquid-to-liquid heat exchanger in a similar manneras described with respect to FIG. 18. As shown in FIG. 19, however, theheat-exchange subsystem 1950 includes a separate pressure reservoir orvessel 1958 containing a liquid (e.g., water), in which the airexpansion occurs. As the gas expands (or is being compressed) in thereservoir 1958, liquid is forced into a liquid to hydraulic fluidcylinder 1901. The liquid (e.g., water) in reservoir 1958 and cylinder1901 is also circulated via a circulator 1952 through a heat exchanger1954, and sprayed back into the vessel 1958 allowing for heat exchangebetween the air expanding (or being compressed) and the liquid. Overall,this embodiment allows for dead-space volume to be filled with anincompressible liquid; thus, the heat-exchanger volume can be large andit can be located anywhere. Likewise, as liquid-to-liquid heatexchangers tend to be more efficient than air-to-liquid heat exchangers,heat transfer may be improved. By adding a separate, larger liquidreservoir 1958, the liquid can be sprayed throughout the entire gasvolume, increasing heat transfer over the set-up shown in FIG. 17.

FIGS. 20A and 20B are schematic diagrams of another alternative systemand method for expedited heat transfer to gas expanding (or beingcompressed) in an open-air staged hydraulic-pneumatic system. In thissetup, the system is arranged to eliminate dead air space and use asimilar type of heat transfer subsystem as described with respect toFIG. 11. Similar to the cylinder 1101 shown in FIG. 11, the cylinder2001 includes a primary gas port 2005, which can be closed via a valveand connected with a pneumatic circuit, or any other pneumaticsource/storage system. The cylinder 2001 further includes a primaryfluid port 2007 that can be closed by a valve. This fluid port connectswith a source of fluid in the hydraulic circuit of the above-describedstorage systems, or any other fluid reservoir. In addition, as the gasis expanded (or being compressed) in the cylinder 2001, the gas is alsocirculated by circulator 2052 through an air-to-liquid heat exchanger2054, which may be a shell-and-tube type with the input 2022 and output2024 from the shell running to an environmental heat exchanger or to asource of process heat, cold water, or other external heat exchangemedium.

As shown in FIG. 20A, a sufficient amount of a liquid (e.g., water) isadded to the pneumatic side 2002 of the cylinder 2001, such that no deadspace is present (e.g., the heat transfer subsystem 2050 (i.e., thecirculator 2052 and heat exchanger 2054 are filled with liquid) when thepiston is fully to the top (e.g., hydraulic side 2004 is filled withhydraulic fluid). The circulator 2052 must be capable of circulatingboth liquid (e.g., water) and air. During the first part of theexpansion, a mix of liquid and air is circulated through the heatexchanger 2054. Because the cylinder 2001 is mounted vertically,however, gravity will tend to empty circulator 2052 of liquid and mostlyair will be circulated during the remainder of the expansion cycle shownin FIG. 20B. Overall, this set-up allows for dead-space volume to befilled with an incompressible liquid and thus the heat exchanger volumecan be large and it can be located anywhere.

FIGS. 21A-21C are schematic diagrams of another alternative system andmethod for expedited heat transfer to gas expanding (or beingcompressed) in an open-air staged hydraulic-pneumatic system. In thissetup, the system is arranged to eliminate dead air space and use asimilar heat transfer subsystem as described with respect to FIG. 11. Inaddition, this set-up uses an auxiliary accumulator 2110 to store andrecover energy from the liquid initially filling an air circulator 2152and a heat exchanger 2154. Similar to the cylinder 1101 shown in FIG.11, the cylinder 2101 includes a primary gas port 2105, which can beclosed via a valve and connected with a pneumatic circuit, or any otherpneumatic source/storage system. The cylinder 2101 further includes aprimary fluid port 2107 a that can be closed by a valve. This fluid port2107 a connects with a source of fluid in the hydraulic circuit of theabove-described storage systems, or any other fluid reservoir. Theauxiliary accumulator 2110 also includes a fluid port 2107 b that can beclosed by a valve and connected to a source of fluid. In addition, asthe gas is expanded (or being compressed) in the cylinder 2101, the gasis also circulated by circulator 2152 through an air to liquid heatexchanger 2154, which may be a shell-and-tube type with the input 2122and output 2124 from the shell running to an environmental heatexchanger or to a source of process heat, cold water, or other externalheat exchange medium.

Additionally, as opposed to the set-up shown in FIGS. 20A and 20B, thecirculator 2152 circulates almost entirely air and not liquid. As shownin FIG. 21A, sufficient liquid (e.g., water) is added to the pneumaticside 2102 of cylinder 2101, such that no dead space is present—e.g., theheat transfer subsystem 2150 (i.e., the circulator 2152 and the heatexchanger 2154) are filled with liquid—when the piston is fully to thetop (e.g., hydraulic side 2104 is filled with hydraulic liquid). InFIGS. 21A-21C, valves shaded black are closed and unshaded valves areopen. During the first part of the expansion, liquid is driven out ofthe circulator 2152 and the heat exchanger 2154, as shown in FIG. 21Bthrough the auxiliary accumulator 2110 and used to produce power. Whenthe auxiliary accumulator 2110 is empty of liquid and full of compressedgas, valves are closed as shown in FIG. 21C and the expansion and aircirculation continues as described above with respect to FIG. 11.Overall, this method allows for dead-space volume to be filled with anincompressible liquid and thus the heat exchanger volume can be largeand it can be located anywhere. Likewise, useful work is extracted whenthe air circulator 2152 and the heat exchanger 2154 are filled withcompressed gas, such that overall efficiency is increased.

FIGS. 22A and 22B are schematic diagrams of another alternative systemand method for expedited heat transfer to gas expanding (or beingcompressed) in an open-air staged hydraulic-pneumatic system. In thissetup, water is sprayed downward into a vertically orientedhydraulic-pneumatic cylinder (accumulator or intensifier) 2201, with ahydraulic side 2203 separated from a pneumatic side 2202 by a moveablepiston 2204. FIG. 22A depicts the cylinder 2201 in fluid communicationwith the heat transfer subsystem 2250 in a state prior to a cycle ofcompressed-air expansion. It should be noted that the air side 2202 ofthe cylinder 2201 is completely filled with liquid, leaving no air space(a circulator 2252 and a heat exchanger 2254 are filled with liquid aswell), when the piston 2204 is fully to the top as shown in FIG. 22A.

Stored compressed gas in pressure vessels, not shown but indicated by2220, is admitted via valve 2221 into the cylinder 2201 through air port2205. As the compressed gas expands into the cylinder 2201, hydraulicfluid is forced out under pressure through fluid port 2207 to theremaining hydraulic system (such as a hydraulic motor as shown anddescribed with respect to FIGS. 1 and 4) as indicated by 2211. Duringexpansion (or compression), heat-exchange liquid (e.g., water) is drawnfrom a reservoir 2230 by a circulator, such as a pump 2252, through aliquid-to-liquid heat exchanger 2254, which may be a shell-and-tube typewith an input 2222 and an output 2224 from the shell running to anenvironmental heat exchanger or to a source of process heat, cold water,or other external heat exchange medium.

As shown in FIG. 22B, the liquid (e.g., water) that is circulated bypump 2252 (at a pressure similar to that of the expanding gas) issprayed (as shown by spray lines 2262) via a spray head 2260 into thepneumatic side 2202 of the cylinder 2201. Overall, this method allowsfor an efficient means of heat exchange between the sprayed liquid(e.g., water) and the air being expanded (or compressed) while usingpumps and liquid to liquid heat exchangers. It should be noted that inthis particular arrangement, the hydraulic pneumatic cylinder 2201 wouldbe oriented vertically, so that the heat-exchange liquid falls withgravity. At the end of the cycle, the cylinder 2201 is reset, and in theprocess, the heat-exchange liquid added to the pneumatic side 2202 isremoved via the pump 2252, thereby recharging reservoir 2230 andpreparing the cylinder 2201 for a successive cycling.

FIG. 22C depicts the cylinder 2201 in greater detail with respect to thespray head 2260. In this design, the spray head 2260 is used much like ashower head in the vertically oriented cylinder. In the embodimentshown, the nozzles 2261 are evenly distributed over the face of thespray head 2260; however, the specific arrangement and size of thenozzles can vary to suit a particular application. With the nozzles 2261of the spray head 2260 evenly distributed across the end-cap area, theentire air volume (pneumatic side 2202) is exposed to the water spray2262. As previously described, the heat-transfer subsystemcirculates/injects the water into the pneumatic side 2202 at a pressureslightly higher than the air pressure and then removes the water at theend of the return stroke at ambient pressure.

As previously discussed, the specific operating parameters of the spraywill vary to suit a particular application. For a specific pressurerange, spray orientation, and spray characteristics, heat-transferperformance can be approximated through modeling. Considering anexemplary embodiment using an 8″ diameter, 10 gallon cylinder with 3000psi air expanding to 300 psi, the water spray flow rates can becalculated for various drop sizes and spray characteristics that wouldbe necessary to achieve sufficient heat transfer to maintain anisothermal expansion. FIG. 22D represents the calculated thermal heattransfer power (in kW) per flow rate (in GPM) for each degree differencebetween the spray liquid and air at 300 and 3000 psi. The lines with theX marks show the relative heat transfer for a regime (Regime 1) wherethe spray breaks up into drops. The calculations assume conservativevalues for heat transfer and no recirculation of the drops, but ratherprovide a conservative estimate of the heat transfer for Regime 1. Thelines with no marks show the relative heat transfer for a regime (Regime2) where the spray remains in coherent jets for the length of thecylinder. The calculations assume conservative values for heat transferand no recirculation after impact, but a conservative estimate of theheat transfer for Regime 2. Considering that an actual spray may be inbetween a jet and pure droplet formation, the two regimes provide aconservative upper bound and fixed lower bound on expected experimentalperformance. Considering a 0.1 kW requirement per gallons per minute(GPM) per ° C., drop sizes under 2 mm provide adequate heat transfer fora given flow rate and jet sizes under 0.1 mm provide adequate heattransfer.

Generally, FIG. 22D represents thermal transfer power levels (kW)achieved, normalized by flow rates required and each Celsius degree oftemperature difference between liquid spray and air, at differentpressures for a spray head (see FIG. 22C) and a vertically-oriented 10gallon, 8″ diameter cylinder. Higher numbers indicate a more efficient(more heat transfer for a given flow rate at a certain temperaturedifference) heat transfer between the liquid spray and the air. Alsoshown graphically is the relative number of holes required to provide ajet of a specific diameter. To minimize the number of spray holesrequired in the spray head requires that the spray break-up intodroplets. The break-up of the spray into droplets versus a coherent jetcan be estimated theoretically using simplifying assumptions on nozzleand fluid dynamics. In general, break-up occurs more predominantly athigher air pressure and higher flow rates (i.e., higher pressure dropacross the nozzle). Break-up at high pressures can be analyzedexperimentally with specific nozzles, geometries, fluids, and airpressures.

Generally, a nozzle size of 0.2 to 2.0 mm is appropriate for highpressure air cylinders (3000 to 300 psi). Flow rates of 0.2 to 1.0liters/min per nozzle are sufficient in this range to provide medium tocomplete spray breakup into droplets using mechanically or laser drilledcylindrical nozzle shapes. For example, a spray head with 250 nozzles of0.9 mm hole diameter operating at 25 gpm is expected to provide over 50kW of heat transfer to 3000 to 300 psi air expanding (or beingcompressed) in a 10 gallon cylinder. Pumping power for such a spray heattransfer implementation was determined to be less than 1% of the heattransfer power. Additional specific and exemplary details regarding theheat transfer subsystem utilizing the spray technology are discussedwith respect to FIGS. 24A and 24B.

FIGS. 23A and 23B are schematic diagrams of another alternative systemand method for expedited heat transfer to gas expanding (or beingcompressed) in an open-air staged hydraulic-pneumatic system. In thissetup, water is sprayed radially into an arbitrarily oriented cylinder2301. The orientation of the cylinder 2301 is not essential to theliquid spraying but is shown as horizontal in FIGS. 23A and 23B. Thehydraulic-pneumatic cylinder (accumulator or intensifier) 2301 has ahydraulic side 2303 separated from a pneumatic side 2302 by a moveablepiston 2304. FIG. 23A depicts the cylinder 2301 in fluid communicationwith the heat-transfer subsystem 2350 in a state prior to a cycle ofcompressed air expansion. It should be noted that no air space ispresent on the pneumatic side 2302 in the cylinder 2301 (e.g., acirculator 2352 and a heat exchanger 2354 are filled with liquid) whenthe piston 2304 is fully retracted (i.e., the hydraulic side 2303 isfilled with liquid) as shown in FIG. 23A.

Stored compressed gas in pressure vessels, not shown in FIGS. 23A, 23Bbut indicated by 2320, is admitted via valve 2321 into the cylinder 2301through air port 2305. As the compressed gas expands into the cylinder2301, hydraulic fluid is forced out under pressure through fluid port2307 to the remaining hydraulic system (such as a hydraulic motor asdescribed with respect to FIGS. 1 and 4) as indicated by arrow 2311.During expansion (or compression), heat-exchange liquid (e.g., water) isdrawn from a reservoir 2330 by a circulator, such as a pump 2352,through a liquid-to-liquid heat exchanger 2354, which may be atube-in-shell setup with an input 2322 and an output 2324 from the shellrunning to an environmental heat exchanger or to a source of processheat, cold water, or other external heat exchange medium. As indicatedin FIG. 23B, the liquid (e.g., water) that is circulated by pump 2352(at a pressure similar to that of the expanding gas) is sprayed (asshown by spray lines 2362) via a spray rod 2360 into the pneumatic side2302 of the cylinder 2301. The spray rod 2360 is shown in this exampleas fixed in the center of the cylinder 2301 with a hollow piston rod2308 separating the heat exchange liquid (e.g., water) from thehydraulic side 2303. As the moveable piston 2304 is moved (for example,leftward in FIG. 23B) forcing hydraulic fluid out of cylinder 2301, thehollow piston rod 2308 extends out of the cylinder 2301 exposing more ofthe spray rod 2360, such that the entire pneumatic side 2302 is exposedto the heat-exchange spray as indicated by spray lines 2362. Overall,this method allows for an efficient means of heat exchange between thesprayed liquid (e.g., water) and the air being expanded (or compressed)while using pumps and liquid-to-liquid heat exchangers. It should benoted that in this particular arrangement, the hydraulic-pneumaticcylinder could be oriented in any manner and does not rely on theheat-exchange liquid falling with gravity. At the end of the cycle, thecylinder 2301 is reset, and in the process, the heat exchange liquidadded to the pneumatic side 2302 is removed via the pump 2352, therebyrecharging reservoir 2330 and preparing the cylinder 2301 for asuccessive cycling.

FIG. 23C depicts the cylinder 2301 in greater detail with respect to thespray rod 2360. In this design, the spray rod 2360 (e.g., a hollowstainless steel tube with many holes) is used to direct the water sprayradially outward throughout the air volume (pneumatic side 2302) of thecylinder 2301. In the embodiment shown, the nozzles 2361 are evenlydistributed along the length of the spray rod 2360; however, thespecific arrangement and size of the nozzles can vary to suit aparticular application. The water can be continuously removed from thebottom of the pneumatic side 2302 at pressure, or can be removed at theend of a return stroke at ambient pressure. This arrangement utilizesthe common practice of center-drilling piston rods (e.g., for positionsensors). As previously described, the heat-transfer subsystem 2350(FIG. 23B) circulates/injects the water into the pneumatic side 2302 ata pressure slightly higher than the air pressure and then removes thewater at the end of the return stroke at ambient pressure.

As previously discussed, the specific operating parameters of the spraywill vary to suit a particular application. For a specific pressurerange, spray orientation, and spray characteristics, heat transferperformance can be approximated through modeling. Again, considering anexemplary embodiment using an 8″ diameter, 10 gallon cylinder with 3000psi air expanding to 300 psi, the water spray flow rates can becalculated for various drop sizes and spray characteristics that wouldbe necessary to achieve sufficient heat transfer to maintain anisothermal expansion. FIG. 23D represents the calculated thermal heattransfer power (in kW) per flow rate (in GPM) for each degree differencebetween the spray liquid and air at 300 and 3000 psi. The lines with theX marks show the relative heat transfer for Regime 1, where the spraybreaks up into drops. The calculations assume conservative values forheat transfer and no recirculation of the drops, but rather provide aconservative estimate of the heat transfer for Regime 1. The lines withno marks show the relative heat transfer for Regime 2, where the sprayremains in coherent jets for the length of the cylinder. Thecalculations assume conservative values for heat transfer and norecirculation after impact, but a conservative estimate of the heattransfer for Regime 2. Considering that an actual spray may be inbetween a jet and pure droplet formation, the two regimes provide aconservative upper bound and fixed lower bound on expected experimentalperformance. Considering a 0.1 kW requirement per gallons per minute(gpm) per ° C., drop sizes under 2 mm provide adequate heat transfer fora given flow rate and jet sizes under 0.1 mm provide adequate heattransfer.

Generally, FIG. 23D represents thermal transfer power levels (kW)achieved, normalized by flow rates required and each Celsius degree oftemperature difference between liquid spray and air, at differentpressures for a spray rod (see FIG. 23C) and a horizontally-oriented 10gallon, 8″ diameter cylinder. Higher numbers indicate a more efficient(more heat transfer for a given flow rate at a certain temperaturedifference) heat transfer between the liquid spray and the air. Alsoshown graphically is the relative number of holes required to provide ajet of a specific diameter. To minimize the number of spray holesrequired in the spray rod requires that the spray break-up intodroplets. The break-up of the spray into droplets versus a coherent jetcan be estimated theoretically using simplifying assumptions on nozzleand liquid dynamics. In general, break-up occurs more prominently athigher air pressure and higher flow rates (i.e., higher pressure dropacross the nozzle). Break-up at high pressures can be analyzedexperimentally with specific nozzles, geometries, fluids, and airpressures.

As discussed above with respect to the spray head arrangement, a nozzlesize of 0.2 to 2.0 mm is appropriate for high pressure air cylinders(3000 to 300 psi). Flow rates of 0.2 to 1.0 liters/min per nozzle aresufficient in this range to provide medium to complete spray breakupinto droplets using mechanically or laser drilled cylindrical nozzleshapes. For example, a spray head with 250 nozzles of 0.9 mm holediameter operating at 25 gpm is expected to provide over 50 kW of heattransfer to 3000 to 300 psi air expanding (or being compressed) in a 10gallon cylinder. Pumping power for such a spray heat transferimplementation may be less than 1% of the heat transfer power.Additional specific and exemplary details regarding the heat transfersubsystem utilizing the spray technology are discussed with respect toFIGS. 24A and 24B.

Generally, for the arrangements shown in FIGS. 22 and 23, theliquid-spray heat transfer may be implemented usingcommercially-available pressure vessels, such as pneumatic andhydraulic/pneumatic cylinders with, at most, minor modifications.Likewise, the heat exchanger may be constructed fromcommercially-available, high-pressure components, thereby reducing thecost and complexity of the overall system. Since the primary heatexchanger area is external of the hydraulic/pneumatic vessel anddead-space volume is filled with an essentially incompressible liquid,the heat exchanger volume may be large and it may be located anywherethat is convenient. In addition, the heat exchanger may be attached tothe vessel with common pipe fittings.

The basic design criteria for the spray heat-transfer subsystem includeminimization of operational energy used (i.e., parasitic loss),primarily related to liquid spray pumping power, while maximizingthermal transfer. While actual heat transfer performance is determinedexperimentally, theoretical analysis indicates the areas where maximumheat transfer for a given pumping power and flow rate of water mayoccur. As heat transfer between the liquid spray and surrounding air isat least partially dependent on surface area, the analysis discussedherein utilized the two spray regimes discussed above: 1) water dropletheat transfer and 2) water jet heat transfer.

In Regime 1, the spray breaks up into droplets, providing a larger totalsurface area. Regime 1 can be considered an upper-bound for surfacearea, and thus heat transfer, for a given set of other assumptions. InRegime 2, the spray remains in a coherent jet or stream, thus providingmuch less surface area for a given volume of water. Regime 2 can beconsidered a lower-bound for surface area and thus heat transfer for agiven set of other assumptions.

For Regime 1, where the spray breaks into droplets for a given set ofconditions, it can be shown that drop sizes of less than 2 mm canprovide sufficient heat transfer performance for an acceptably low flowrate (e.g., <10 gpm ° C./kW), as shown in FIG. 24A. FIG. 24A representsthe flow rates required for each Celsius degree of temperaturedifference between liquid spray droplets and air at different pressuresto achieve one kilowatt of heat transfer. Lower numbers indicate a moreefficient (lower flow rate for given amount of heat transfer at acertain temperature difference) heat transfer between the liquid spraydroplets and the air. For the given set of conditions illustrated inFIG. 24A, drop diameters below about 2 mm are desirable. FIG. 24B is anenlarged portion of the graph of FIG. 24A and represents that for thegiven set of conditions illustrated, drop diameters below about 0.5 mmno longer provide additional heat transfer benefit for a given flowrate.

As drop size continues to become smaller, eventually the terminalvelocity of the drop becomes small enough (e.g., <100 microns) that thedrops fall too slowly to cover the entire cylinder volume. Thus, for thegiven set of conditions illustrated here, drop sizes between about 0.1and 2.0 mm may be considered as preferred for maximizing heat transferwhile minimizing pumping power, which increases with increasing flowrate. A similar analysis can be performed for Regime 2, where liquidspray remains in a coherent jet. Higher flow rates and/or narrowerdiameter jets are generally needed to provide similar heat transferperformance.

FIG. 25 is a detailed schematic diagram of a cylinder design for usewith any of the herein described systems for energy storage and recoveryusing compressed gas. In particular, the cylinder 2501 depicted inpartial cross-section in FIG. 25 includes a spray head arrangement 2560similar to that described with respect to FIG. 22, where water issprayed downward into a vertical cylinder. As shown, the verticallyoriented hydraulic-pneumatic cylinder 2501 has a hydraulic side 2503separated from a pneumatic side 2502 by a moveable piston 2504. Thecylinder 2501 also includes two end caps (e.g., machined steel blocks)2563, 2565, mounted on either end of a honed cylindrical tube 2561,typically attached via tie rods or other well-known mechanical means.The piston 2504 is slidably disposed in and sealingly engaged with thetube 2561 via seals 2567. End cap 2565 is machined with single ormultiple ports 2585, which allow for the flow of hydraulic fluid. Endcap 2563 is machined with single or multiple ports 2586, which can admitair and/or heat-exchange fluid. The ports 2585, 2586 shown have threadedconnections; however, other types of ports/connections are contemplatedand within the scope of the invention (e.g., flanged).

Also illustrated is an optional piston rod 2570 that may be attached tothe moveable piston 2504, allowing for position measurement via adisplacement transducer 2574 and piston damping via an external cushion2575, as necessary. The piston rod 2570 moves into and out of the second(e.g., hydraulic) side 2503 through a machined hole with a rod seal2572. The spray head 2560 in this illustration is inset within the endcap 2563 and attached to a heat-exchange liquid (e.g., water) port 2571via, for example, blind retaining fasteners 2573. Other mechanicalfastening means are contemplated and within the scope of the invention.

FIG. 26 is a detailed schematic diagram of a cylinder design for usewith any of the herein described systems for energy storage and recoveryusing compressed gas. In particular, the cylinder 2601 depicted inpartial cross-section in FIG. 26 includes a spray rod arrangement 2660similar to that described with respect to FIG. 23, where water issprayed radially via an installed spray rod into an arbitrarily-orientedcylinder. As shown, the arbitrarily-oriented hydraulic-pneumaticcylinder 2601 includes a second (e.g., hydraulic) side 2603 separatedfrom a first (e.g., pneumatic) side 2602 by a moveable piston 2604. Thecylinder 2601 includes two end caps (e.g., machined steel blocks) 2663,2665, mounted on either end of a honed cylindrical tube 2661, typicallyattached via tie rods or other well-known mechanical means. The piston2604 is slidably disposed in and sealingly engaged with the tube 2661via seals 2667. End cap 2665 is machined with single or multiple ports2685, which allow for the flow of hydraulic fluid. End cap 2663 ismachined with single or multiple ports 2686, which may admit air and/orheat exchange liquid. The ports 2685, 2686 shown have threadedconnections; however, other types of ports/connections are contemplatedand within the scope of the invention (e.g., flanged).

A hollow piston rod 2608 is attached to the moveable piston 2604 andslides over the spray rod 2660 that is fixed to and oriented coaxiallywith the cylinder 2601. The spray rod 2660 extends through a machinedhole 2669 in the piston 2604. The piston 2604 is configured to movefreely along the length of the spray rod 2660. As the moveable piston2604 moves towards end cap 2665, the hollow piston rod 2608 extends outof the cylinder 2601, exposing more of the spray rod 2660, such that theentire pneumatic side 2602 is exposed to heat-exchange spray (see, forexample, FIG. 23B). The spray rod 2660 in this illustration is attachedto the end cap 2663 and in fluid communication with aheat-exchange-liquid port 2671. As shown in FIG. 26, the port 2671 ismechanically coupled to and sealed with the end cap 2663; however, theport 2671 could also be a threaded connection machined in the end cap2663. The hollow piston rod 2608 also allows for position measurementvia displacement transducer 2674 and piston damping via an externalcushion 2675. As shown in FIG. 26, the piston rod 2608 moves into andout of the hydraulic side 2603 through a machined hole with rod seal2672.

It should be noted that the heat-transfer subsystems discussed abovewith respect to FIGS. 9-13 and 15-23 may also be used in conjunctionwith the high-pressure gas storage systems (e.g., storage tanks 902) tothermally condition the pressurized gas stored therein, as shown inFIGS. 27 and 28. Generally, these systems are arranged and operate inthe same manner as described above.

FIG. 27 depicts the use of a heat transfer subsystem 2750 in conjunctionwith a gas storage system 2701 for use with the compressed gas energystorage systems described herein, to expedite transfer of thermal energyto, for example, the compressed gas prior to and during expansion.Compressed air from the pressure vessels (2702 a-2702 d) is circulatedthrough a heat exchanger 2754 using an air pump 2752 operating as acirculator. The air pump 2752 operates with a small pressure changesufficient for circulation, but within a housing that is able towithstand high pressures. The air pump 2752 circulates the high-pressureair through the heat exchanger 2754 without substantially increasing itspressure (e.g., a 50 psi increase for 3,000 psi air). In this way, thestored compressed air may be pre-heated (or pre-cooled) by opening valve2704 with valve 2706 closed and heated during expansion or cooled duringcompression by closing 2704 and opening 2706 (which may also placeheat-transfer subsystem 2750 in fluid communication with an energystorage and recovery system). The heat exchanger 2754 may be any sort ofstandard heat-exchanger design; illustrated here is a tube-in-shell typeheat exchanger with high-pressure air inlet and outlet ports 2721 a and2721 b, and low-pressure shell water ports 2722 a and 2722 b.

FIG. 28 depicts the use of a heat-transfer subsystem 2850 in conjunctionwith a gas storage system 2801 for use with the compressed gas in energystorage systems described herein, to expedite transfer of thermal energyto the compressed gas prior to and during expansion. In this embodiment,thermal energy transfer to and from the stored compressed gas inpressure vessels (2802 a, 2802 b) is expedited through a watercirculation scheme using a water pump 2852 and heat exchanger 2854. Thewater pump 2852 operates with a small pressure change sufficient forcirculation and spray, but within a housing that is able to withstandhigh pressures. The water pump 2852 circulates high-pressure waterthrough heat exchanger 2854 and sprays the water into pressure vessels2802 a, 2802 b without substantially increasing its pressure (e.g., a100 psi increase for circulating and spraying within 3,000 psi storedcompressed air). In this way, the stored compressed air may bepre-heated (or pre-cooled) using a water circulation and spraying methodthat also allows for active water monitoring of the pressure vessels2802.

The spray heat exchange may occur as pre-heating prior to expansionand/or pre-cooling prior to compression in the system when valve 2806 isopened. The heat exchanger 2854 may be any sort of standard heatexchanger design; illustrated here is a tube-in-shell type heatexchanger with high-pressure water inlet and outlet ports 2821 a and2821 b and low-pressure shell water ports 2822 a and 2822 b. Asliquid-to-liquid heat exchangers tend to be more efficient thanair-to-liquid heat exchangers, heat exchanger size may be reduced and/orheat transfer may be improved by use of the liquid to liquid heatexchanger. Heat exchange within the pressure vessels 2802 a, 2802 b isexpedited by active spraying of the liquid (e.g., water) into thepressure vessels 2802.

As shown in FIG. 28, a perforated spray rod 2811 a, 2811 b is installedwithin each pressure vessel 2802 a, 2802 b. The water pump 2852increases the water pressure above the vessel pressure such that wateris actively circulated and sprayed out of rods 2811 a and 2811 b, asshown by arrows 2812 a, 2812 b. After spraying through the volume of thepressure vessels 2802, the water settles to the bottom of the vessels2802 a, 2802 b (forming pools 2813 a, 2813 b) and is then removedthrough a drainage port 2814 a, 2814 b. The water may be circulatedthrough the heat exchanger 2854 as part of the closed-loop watercirculation and spray system.

Alternative systems and methods for energy storage and recovery aredescribed with respect to FIGS. 29-44. These systems and methods aresimilar to the energy storage and recovery systems described above, butuse a variety of mechanical means coupled to different types ofcylinders. Such systems may include (a) distinct pneumatic and hydraulicfree-piston cylinders, mechanically coupled to each other by amechanical boundary mechanism, rather than a single pneumatic-hydrauliccylinder, such as an intensifier, or (b) pneumatic free-piston cylinderscoupled to electrical machines by mechanical boundary mechanisms orsubsystems rather than by hydraulic subsystems. Systems employingdistinct pneumatic and hydraulic free-piston cylinders allow theheat-transfer subsystems for conditioning the gas being expanded (orcompressed) to be separated from the hydraulic circuit. By mechanicallycoupling one or more pneumatic cylinders and/or one or more hydrauliccylinders so as to add (or share) forces produced by (or acting on) thecylinders, the hydraulic pressure range may be narrowed, allowing moreefficient operation of the hydraulic motor/pump. Systems couplingpneumatic cylinders to electrical machines by mechanical means (e.g.,coupling of cylinder rods to linear generators, coupling of cylinderrods to crankshafts that are in turn coupled to rotary electricalmachines) allow the omission of hydraulic cylinders and pump/motors andefficient conversion of the elastic potential energy of compressed gasto electrical energy or the reverse.

The systems and methods described with respect to FIGS. 29-31 generallyoperate on the principle of transferring mechanical energy between twoor more cylinder assemblies using a mechanical boundary mechanism tomechanically couple the cylinder assemblies and translate the linearmotion produced by one cylinder assembly to the other cylinder assembly.In one embodiment, the linear motion of the first cylinder assembly isthe result of a gas expanding in one chamber of the cylinder and movinga piston within the cylinder. The translated linear motion in the secondcylinder assembly is converted into a rotary motion of a hydraulicmotor, as the linear motion of the piston in the second cylinderassembly drives a fluid out of the cylinder and to the hydraulic motor.The rotary motion is converted to electricity by using a rotary electricgenerator.

The basic operation of a compressed-gas energy storage system for usewith the cylinder assemblies described with respect to FIGS. 29-31 is asfollows. The gas is expanded into a cylindrical chamber (i.e., thepneumatic cylinder assembly) containing a piston or other mechanism thatseparates the gas on one side of the chamber from the other, therebypreventing gas movement from one chamber to the other while allowing thetransfer of force/pressure from one chamber to the other. A shaftattached to and extending from the piston is attached to anappropriately sized mechanical boundary mechanism that communicatesforce to the shaft of a hydraulic cylinder, also divided into twochambers by a piston. In one embodiment, the active area of the pistonof the hydraulic cylinder is smaller than the area of the pneumaticpiston, resulting in an intensification of pressure (i.e., the ratio ofthe pressure in the chamber undergoing compression in the hydrauliccylinder to the pressure in the chamber undergoing expansion in thepneumatic cylinder) proportional to the difference in piston areas. Thehydraulic fluid pressurized in the hydraulic cylinder may be used toturn a hydraulic motor/pump, either fixed-displacement orvariable-displacement, whose shaft may be affixed to that of a rotaryelectric motor/generator in order to produce electricity. Heat-transfersubsystems, such as those described above, may be combined with thesecompressed-gas energy storage systems to expand/compress the gassubstantially isothermally to achieve maximum efficiency.

The systems and methods described with respect to FIGS. 32-44 generallyoperate on a similar principle of transferring mechanical energy to orfrom one or more pneumatic cylinder assemblies using a mechanicalboundary mechanism to mechanically couple the one or more cylinderassemblies to electrical machines. In some embodiments, the linearmotion produced by the one or more cylinder assemblies is translated tothe mover of a linear electrical machine (motor/generator) by a suitablelinkage, generating electricity. In other embodiments, the linear motionproduced by the one or more cylinder assemblies is converted to rotarymotion by a crankshaft assembly and may be mechanically transmittedtherefrom to a rotary electrical machine (motor/generator), generatingelectricity. In various embodiments, energy may be transferred to,rather than from, the one or more pneumatic cylinder assemblies bysuitable operation of the electrical and other components of suchcompressed-gas energy storage systems. Heat-transfer subsystems, such asthose described above, may be combined with these compressed-gas energystorage systems to expand/compress the gas substantially isothermally toachieve maximum efficiency.

FIGS. 29A and 29B are schematic diagrams of a system for usingcompressed gas to operate two series-connected, double-acting pneumaticcylinders coupled to a single double-acting hydraulic cylinder to drivea hydraulic motor/generator to produce electricity (i.e., gasexpansion). If the motor/generator is operated as a motor rather than asa generator, the identical mechanism maaay employ electricity to producepressurized stored gas (i.e.; gas compression). FIG. 29A depicts thesystem in a first phase of operation and FIG. 29B depicts the system ina second phase of operation, where the high- and low-pressure sides ofthe pneumatic cylinders are reversed and the direction of hydraulicmotor shaft motion is reversed, as discussed in greater detailhereinbelow.

Generally, the expansion of the gas occurs in multiple stages, using thelow- and high-pressure pneumatic cylinders. For example, in the case oftwo pneumatic cylinders, as shown in FIG. 29A, high-pressure gas isexpanded in the high-pressure pneumatic cylinder from a maximum pressure(e.g., 3000 psi) to some mid-pressure (e.g., 300 psi); then thismid-pressure gas is further expanded (e.g., 300 psi to 30 psi) in theseparate low-pressure cylinder. These two stages are coupled to thecommon mechanical boundary mechanism that communicates force to theshaft of the hydraulic cylinder. When each of the two pneumatic pistonsreaches the limit of its range of motion, valves or other mechanisms maybe adjusted to direct higher-pressure gas to, and vent lower-pressuregas from, the cylinder's two chambers so as to produce piston motion inthe opposite direction. In double-acting devices of this type, there isno withdrawal stroke or unpowered stroke, i.e., the stroke is powered inboth directions.

The chambers of the hydraulic cylinder being driven by the pneumaticcylinders may be similarly adjusted by valves or other mechanisms toproduce pressurized hydraulic fluid during the return stroke. Moreover,check valves or other mechanisms may be arranged so that regardless ofwhich chamber of the hydraulic cylinder is producing pressurized fluid,a hydraulic motor/pump is driven in the same direction of rotation bythat fluid. The rotating hydraulic motor/pump and electricalmotor/generator in such a system do not reverse their direction ofrotation when piston motion reverses, so that with the addition of ashort-term-energy-storage device, such as a flywheel, the resultingsystem may be made to generate electricity continuously (i.e., withoutinterruption during piston reversal).

As shown in FIG. 29A, the system 2900 consists of a first pneumaticcylinder 2901 divided into two chambers 2902, 2903 by a piston 2904. Thecylinder 2901, which is shown in a horizontal orientation in thisillustrative embodiment, but may be arbitrarily oriented, has one ormore gas circulation ports 2905 that are connected via piping 2906 andvalves 2907, 2908 to a compressed-gas reservoir or storage system 2909.The pneumatic cylinder 2901 is connected via piping 2910, 2911 andvalves 2912, 2913 to a second pneumatic cylinder 2914 operating at alower pressure than the first. Both cylinders 2901, 2914 aredouble-acting and are attached in series (pneumatically) and in parallel(mechanically). Series attachment of the two cylinders 2901, 2914 meansthat gas from the lower-pressure chamber of the high-pressure cylinder2901 is directed to the higher-pressure chamber of the low-pressurecylinder 2914.

Pressurized gas from the reservoir 2909 drives the piston 2904 of thedouble-acting high-pressure cylinder 2901. In the state of operationshown in FIG. 29A, intermediate-pressure gas from the lower-pressurechamber 2903 of the high-pressure cylinder 2901 is conveyed through avalve 2912 to the higher-pressure chamber 2915 of the lower-pressurecylinder 2914. Gas is conveyed from the lower-pressure chamber 2916 ofthe lower-pressure cylinder 2914 through a valve 2917 to a vent 2918.One function of this arrangement is to reduce the range of pressuresover which the cylinders jointly operate.

The piston shafts 2919, 2920 of the two cylinders 2914, 2901 act jointlyto move the mechanical boundary mechanism 2921 in the directionindicated by the arrow 2922. The mechanical boundary mechanism 2921 isalso connected to the piston shaft 2923 of the hydraulic cylinder 2924.The piston 2925 of the hydraulic cylinder 2924, impelled by themechanical boundary mechanism 2921, compresses hydraulic fluid in thechamber 2926. This pressurized hydraulic fluid is conveyed throughpiping 2927 to an arrangement of check valves 2928 that allows the fluidto flow in one direction (shown by the arrows) through a hydraulicmotor/pump, either fixed-displacement or variable-displacement, whoseshaft drives an electric motor/generator. For convenience, thecombination of hydraulic pump/motor and electric motor/generator isshown as a single hydraulic power unit 2929. Hydraulic fluid at lowerpressure is conducted from the output of the hydraulic motor/pump 2929to the lower-pressure chamber 2930 of the hydraulic cylinder 2924through piping 2933 and a hydraulic circulation port 2931.

Reference is now made to FIG. 29B, which depicts the system 2900 of FIG.29A in a second operating state, where valves 2907, 2913, and 2932 areopen and valves 2908, 2912, and 2917 are closed. In this state, gasflows from the high-pressure reservoir 2909 through valve 2907 intochamber 2903 of the high-pressure pneumatic cylinder 2901.Lower-pressure gas is vented from the other chamber 2902 via valve 2913to chamber 2916 of the lower-pressure pneumatic cylinder 2914. Thepiston shafts 2919, 2920 of the two cylinders act jointly to move themechanical boundary mechanism 2921 in the direction indicated by thearrow 2922. The mechanical boundary mechanism 2921 translates themovement of shafts 2919, 2920 to the piston shaft 2923 of the hydrauliccylinder 2924. The piston 2925 of the hydraulic cylinder 2924, impelledby the mechanical boundary mechanism 2921, compresses hydraulic fluid inthe chamber 2930. This pressurized hydraulic fluid is conveyed throughpiping 2933 to the aforementioned arrangement of check valves 2928 andthe hydraulic power unit 2929. Hydraulic fluid at a lower pressure isconducted from the output of the hydraulic power unit 2929 to thelower-pressure chamber 2926 of the hydraulic cylinder 2924 through ahydraulic circulation port 2935.

As shown in FIGS. 29A and 29B, the stroke volumes of the two chambers ofthe hydraulic cylinder 2924 differ by the volume of the shaft 2923. Theresulting imbalance in fluid volumes expelled from the cylinder 2924during the two stroke directions shown in FIGS. 29A and 29B may becorrected either by a pump (not shown) or by extending the shaft 2923through the entire length of both chambers 2926, 2930 of the cylinder2924, so that the two stroke volumes are equal.

As previously discussed, the efficiency of the various energy storageand recovery systems described herein can be increased by using aheat-transfer subsystem. Accordingly, the system 2900 shown in FIGS. 29Aand 29B may include a heat-transfer subsystem 2950 similar to thosedescribed above. Generally, the heat transfer subsystem 2950 includes afluid circulator 2952 and a heat exchanger 2954. The subsystem 2950 alsoincludes two directional control valves 2956, 2958 that selectivelyconnect the subsystem 2950 to one or more chambers of the pneumaticcylinders 2901, 2914 via pairs of gas ports on the cylinders 2901, 2914identified as A and B. For example, the valves 2956, 2958 may bepositioned to place the subsystem 2950 in fluidic communication withchamber 2903 during gas expansion therein, so as to thermally conditionthe gas expanding in the chamber 2903. The gas may be thermallyconditioned by any of the previously described methods, for example, thegas from the selected chamber may be circulated through the heatexchanger. Alternatively, a heat-exchange liquid may be circulatedthrough the selected gas chamber and any of the previously describedspray arrangements for heat exchange may be used. During expansion (orcompression), a heat-exchange liquid (e.g., water) may be drawn from areservoir (not shown, but similar to those described above with respectto FIG. 22) by the circulator 2954, circulated through aliquid-to-liquid version of the heat exchanger 2954, which may be ashell-and-tube type with an input 2962 and an output 2960 from the shellrunning to an environmental heat exchanger or to a source of processheat, cold water, or other external heat exchange medium.

FIGS. 30A-30D depict an alternative embodiment of the system of FIG. 29modified to have a single pneumatic cylinder and two hydrauliccylinders. A decreased range of hydraulic pressures, with consequentlyincreased motor/pump and motor/generator efficiencies, may be obtainedby using two or more hydraulic cylinders. As shown, these two cylindersare connected to the aforementioned mechanical boundary mechanism forcommunicating force with the pneumatic cylinder. The chambers of the twohydraulic cylinders are attached to valves, lines, and other mechanismsin such a manner that either cylinder can, with appropriate adjustments,be set to present no resistance as its shaft is moved (i.e., compress nofluid).

FIG. 30A depicts the system in a state of operation where both hydraulicpistons are compressing hydraulic fluid. One effect of this arrangementis to decrease the range of hydraulic pressures delivered to thehydraulic motor as the force produced by the pressurized gas in thepneumatic cylinder decreases with expansion and as the pressure of thegas stored in the reservoir decreases. FIG. 30B depicts the system in aphase of operation where only one of the hydraulic cylinders iscompressing hydraulic fluid. FIG. 30C depicts the system in a phase ofoperation where the high- and low-pressure sides of the hydrauliccylinders are reversed along with the direction of shafts and only thesmaller-bore hydraulic cylinder is compressing hydraulic fluid. FIG. 30Ddepicts the system in a phase of operation similar to FIG. 30C, but withboth hydraulic cylinders compressing hydraulic fluid.

The system 3000 shown in FIG. 30A is similar to system 2900 describedabove and includes a single double-acting pneumatic cylinder 3001 andtwo double-acting hydraulic cylinders 3024 a, 3024 b, where onehydraulic cylinder 3024 a has a larger bore than the other cylinder 3024b. In the state of operation shown, pressurized gas from the reservoir3009 enters one chamber 3002 of the pneumatic cylinder 3001 and drives apiston 3005 slidably disposed in the pneumatic cylinder 3001.Low-pressure gas from the other chamber 3003 of the pneumatic cylinder3001 is conveyed through a valve 3007 to a vent 3008. A shaft 3019extending from the piston 3005 disposed in the pneumatic cylinder 3001moves a mechanically coupled mechanical boundary mechanism 3021 in thedirection indicated by the arrow 3022. The mechanical boundary mechanism3021 is also connected to the piston shafts 3023 a, 3023 b of thedouble-acting hydraulic cylinders 3024 a, 3024 b.

In the current state of operation shown, valves 3014 a and 3014 b permitfluid to flow to hydraulic power unit 3029. Pressurized fluid from bothcylinders 3024 a, 3024 b is conducted via piping 3015 to an arrangementof check valves 3028 and a hydraulic pump/motor connected to amotor/generator, thereby producing electricity. Hydraulic fluid at alower pressure is conducted from the output of the hydraulic motor/pumpto the lower-pressure chambers 3016 a, 3016 b of the hydraulic cylinders3024 a, 3024 b. The fluid in the high-pressure chambers 3026 a, 3026 bof the two hydraulic cylinders 3024 a, 3024 b is at a single pressure,and the fluid in the low-pressure chambers 3016 a, 3016 b is also at asingle pressure. In effect, the two cylinders 3024 a, 3024 b act as asingle cylinder whose piston area is the sum of the piston areas of thetwo cylinders and whose operating pressure, for a given driving forcefrom the pneumatic piston 3001, is proportionately lower than that ofeither hydraulic cylinder acting alone.

Reference is now made to FIG. 30B, which shows another state ofoperation of the system 3000 of FIG. 30A. The action of the pneumaticcylinder 3001 and the direction of motion of all pistons is the same asin FIG. 30A. In the state of operation shown, formerly closed valve 3033is opened to permit fluid to flow freely between the two chambers 3016a, 3026 a of the larger-bore hydraulic cylinder 3024 a, therebypresenting minimal resistance to the motion of its piston 3025 a.Pressurized fluid from the smaller-bore cylinder 3024 b is conducted viapiping 3015 to the aforementioned arrangement of check valves 3028 andthe hydraulic power unit 3029, thereby producing electricity. Hydraulicfluid at a lower pressure is conducted from the output of the hydraulicpower unit 3029 to the lower-pressure chamber 3016 b of the smaller borehydraulic cylinder 3024 b. In effect, the acting hydraulic cylinder 3024b, having a smaller piston area, provides a higher hydraulic pressurefor a given force acting on the mechanically coupled boundary mechanism3021 than in the state shown in FIG. 30A, where both hydraulic cylinders3024 a, 3024 b were acting, with a larger effective piston area. Throughvalve actuations disabling one of the hydraulic cylinders, a narrowedhydraulic fluid pressure range is obtained.

Reference is now made to FIG. 30C, which shows another state ofoperation of the system 3000 of FIGS. 30A and 30B. In the state ofoperation shown, pressurized gas from the reservoir 3009 enters chamber3003 of the pneumatic cylinder 3001, driving its piston 3005.Low-pressure gas from the other side 3002 of the pneumatic cylinder 3001is conveyed through a valve 3035 to the vent 3008. The action of themechanical boundary mechanism 3021 on the pistons 3023 a, 3023 b of thehydraulic cylinders 3024 a, 3024 b is in the opposite direction as thatshown in FIG. 30B, as indicated by arrow 3022.

As in FIG. 30A, valves 3014 a, 3014 b are open and permit fluid to flowto the hydraulic power unit 3029. Pressurized fluid from both hydrauliccylinders 3024 a, 3024 b is conducted via piping 3015 to theaforementioned arrangement of check valves 3028 and the hydraulic powerunit 3029, thereby producing electricity. Hydraulic fluid at a lowerpressure is conducted from the output of the hydraulic power unit 3029to the lower-pressure chambers 3026 a, 3026 b of the hydraulic cylinders3024 a, 3024 b. The fluid in the high-pressure chambers 3016 a, 3016 bof the two hydraulic cylinders 3024 a, 3024 b is at a single pressure,and the fluid in the low-pressure chambers 3026 a, 3026 b is also at asingle pressure. In effect, the two hydraulic cylinders 3024 a, 3024 bact as a single cylinder whose piston area is the sum of the pistonareas of the two cylinders and whose operating pressure, for a givendriving force from the pneumatic piston 3001, is proportionately lowerthan that of either hydraulic cylinder 3024 a, 3024 b acting alone.

Reference is now made to FIG. 30D, which shows another state ofoperation of the system 3000 of FIGS. 30A-30C. The action of thepneumatic cylinder 3001 and the direction of motion of all movingpistons is the same as in FIG. 30C. In the state of operation shown,formerly closed valve 3033 is opened to permit fluid to flow freelybetween the two chambers 3026 a, 3016 a of the larger bore hydrauliccylinder 3024 a, thereby presenting minimal resistance to the motion ofits piston 3025 a. Pressurized fluid from the smaller-bore cylinder 3024b is conducted via piping 3015 to the aforementioned arrangement ofcheck valves 3028 and the hydraulic power unit 3029, thereby producingelectricity. Hydraulic fluid at a lower pressure is conducted from theoutput of the hydraulic motor/pump to the lower-pressure chamber 3026 bof the smaller-bore hydraulic cylinder 3024 b. In effect, the actinghydraulic cylinder 3024 b, having a smaller piston area, provides ahigher hydraulic pressure for a given force than the state shown in FIG.30C, where both cylinders were acting with a larger effective pistonarea. Through valve actuations disabling one of the hydraulic cylinders,a narrowed hydraulic fluid pressure range is obtained.

Additional valving may be added to cylinder 3024 b such that it could bedisabled to provide another effective hydraulic piston area (consideringthat 3024 a and 3024 b are not the same diameter cylinders) to somewhatfurther reduce the hydraulic fluid range for a given pneumatic pressurerange. Likewise, additional hydraulic cylinders and valve arrangementsmay be added to substantially further reduce the hydraulic fluid rangefor a given pneumatic pressure range.

The operation of the exemplary system 3000 described above, where two ormore hydraulic cylinders are driven by a single pneumatic cylinder, isas follows. Assuming that a quantity of high-pressure gas has beenintroduced into one chamber of that single pneumatic cylinder, as thegas begins to expand, moving the piston, force is communicated by thepiston shaft and the mechanical boundary mechanism to the piston shaftsof the two hydraulic cylinders. At any point during the expansion phase,the hydraulic pressure will be equal to the force divided by the actinghydraulic piston area. At the beginning of a stroke, when the gas in thepneumatic cylinder has only begun to expand, it is producing a maximumforce; this force (ignoring frictional losses) acts on the combinedtotal piston area of the hydraulic cylinders, producing a certainhydraulic output pressure, HP_(max).

As the gas in the pneumatic cylinder continues to expand, it exerts adecreasing force. Consequently, the pressure developed in thecompression chamber of the active cylinders decreases. At a certainpoint in the process, the valves and other mechanisms attached to one ofthe hydraulic cylinders is adjusted so that fluid can flow freelybetween its two chambers and thus offer no resistance to the motion ofthe piston (again ignoring frictional losses). The effective piston areadriven by the force developed by the pneumatic cylinder thus decreasesfrom the piston area of both hydraulic cylinders to the piston area ofone of the hydraulic cylinders. With this decrease of area comes anincrease in output hydraulic pressure for a given force. If thisswitching point is chosen carefully, the hydraulic output pressureimmediately after the switch returns to HP_(max). For an example wheretwo identical hydraulic cylinders are used, the switching pressure wouldbe at the half pressure point.

As the gas in the pneumatic cylinder continues to expand, the pressuredeveloped by the hydraulic cylinder decreases. As the pneumatic cylinderreaches the end of its stroke, the force developed is at a minimum andso is the hydraulic output pressure, HP_(min). For an appropriatelychosen ratio of hydraulic cylinder piston areas, the hydraulic pressurerange HR=HP_(max)/HP_(min) achieved using two hydraulic cylinders willbe the square root of the range HR achieved with a single hydrauliccylinder. The proof of this assertion is as follows.

Let a given output hydraulic pressure range HR₁ from high pressureHP_(max) to low pressure HP_(min), namely HR₁=HP_(max)/HP_(min), besubdivided into two pressure ranges of equal magnitude HR₂. The firstrange is from HP_(max) down to some intermediate pressure HP₁ and thesecond is from HP_(I) down to HP_(min). Thus,HR₂=HP_(max)/HP_(I)=HP_(I)/HP_(min). From this identity of ratios,HP_(I)=(HP_(max)/HP_(min))^(1/2). Substituting for HP_(I) inHR₂=HP_(max)/HP_(I), we obtainHR₂=HP_(max)/(HP_(max)/HP_(min))^(1/2)=(HP_(max)HP_(min))^(1/2)=HR₁^(1/2).

Since HP_(max) is determined (for a given maximum force developed by thepneumatic cylinder) by the combined piston areas of the two hydrauliccylinders (HA₁+HA₂), whereas HP_(I) is determined jointly by the choiceof when (i.e., at what force level, as force declines) to deactivate thesecond cylinder and by the area of the single acting cylinder HA₁, it ispossible to choose the switching force point and HA₁ so as to producethe desired intermediate output pressure HP_(I). It can be similarlyshown that with appropriate cylinder sizing and choice of switchingpoints, the addition of a third cylinder/stage will reduce the operatingpressure range as the cube root, and so forth. In general, Nappropriately sized cylinders may reduce an original operating pressurerange HR₁ to HR₁ ^(1/N).

In addition, for a system using multiple pneumatic cylinders (i.e.,dividing the air expansion into multiple stages), the hydraulic pressurerange may be further reduced. For M appropriately sized pneumaticcylinders (i.e., pneumatic air stages) for a given expansion, theoriginal pneumatic operating pressure range PR₁ of a single stroke maybe reduced to PR₁ ^(1/M). Since for a given hydraulic cylinderarrangement the output hydraulic pressure range is directly proportionalto the pneumatic operating pressure range for each stroke,simultaneously combining M pneumatic cylinders with N hydrauliccylinders may realize a pressure range reduction to the 1/(N×M) power,that is, may reduce an original operating pressure range HR₁ to HR₁^(1/NM).

Furthermore, the system 3000 shown in FIGS. 30A-30D may also include aheat transfer subsystem 3050 similar to those described above.Generally, the heat transfer subsystem 3050 includes a fluid circulator3052 and a heat exchanger 3054. The subsystem 3050 also includes twodirectional control valves 3056, 3058 that selectively connect thesubsystem 3050 to one or more chambers of the pneumatic cylinder 3001via pairs of gas ports on the cylinder 3001 identified as A and B. Forexample, the valves 3056, 3058 may be positioned to place the subsystem3050 in fluidic communication with chamber 3003 during gas expansiontherein, so as to thermally condition the gas expanding in the chamber3003. The gas may be thermally conditioned by any of the previouslydescribed methods. For example, during expansion (or compression), aheat exchange liquid (e.g., water) may be drawn from a reservoir (notshown, but similar to those described above with respect to FIG. 22) bythe circulator 3054, circulated through a liquid-to-liquid version ofthe heat exchanger 3054, which may be a shell and tube type with aninput 3060 and an output 3062 from the shell running to an environmentalheat exchanger or to a source of process heat, cold water, or otherexternal heat exchange medium.

FIGS. 31A-31C depict an alternative embodiment of the system of FIG. 30,where the two side-by-side hydraulic cylinders have been replaced by twotelescoping hydraulic cylinders. The effect of this arrangement is todecrease the range of hydraulic pressures delivered to the hydraulicmotor as the force produced by the pressurized gas in the pneumaticcylinder decreases with expansion and as the pressure of the gas storedin the reservoir decreases. FIG. 31A depicts the system in a phase ofoperation where only the outer, larger-bore hydraulic cylinder iscompressing hydraulic fluid. FIG. 31B depicts the system in a phase ofoperation where the outer-cylinder piston has moved to its limit in thedirection of motion and is no longer compressing hydraulic fluid and theinner, smaller-bore cylinder is compressing hydraulic fluid. FIG. 31Cdepicts the system in a phase of operation where the direction of themotion of the cylinders and motor are reversed; the inner, smaller-borecylinder is acting as the shaft of the outer, larger-bore cylinder; andonly the outer, larger-bore cylinder is compressing hydraulic fluid.

The system 3100 shown in FIG. 31A is similar to those described aboveand includes a single double-acting pneumatic cylinder 3101 and twodouble-acting hydraulic cylinders 3124 a, 3124 b, where one cylinder3124 b is telescopically disposed inside the other cylinder 3124 a. Inthe state of operation shown, pressurized gas from the reservoir 3109enters a chamber 3102 of the pneumatic cylinder 3101 and drives a piston3105 slidably disposed with the pneumatic cylinder 3101. Low-pressuregas from the other chamber 3103 of the pneumatic cylinder 3101 isconveyed through a valve 3107 to a vent 3108. A shaft 3119 extendingfrom the piston 3105 disposed in the pneumatic cylinder 3101 moves amechanically coupled mechanical boundary mechanism 3121 in the directionindicated by the arrow 3122. The mechanical boundary mechanism 3121 isconnected to the piston shaft 3123 of the hydraulic cylinder 3124 b. Theentire smaller bore cylinder 3124 b acts as the shaft 3123 of the largerpiston 3125 a of the larger bore hydraulic cylinder 3124 a; therefore,the mechanical boundary mechanism 3122 is coupled to hydraulic cylinder3124 a via its coupling to cylinder 3124 b via shaft 3123.

In the state of operation shown, the entire smaller-bore cylinder 3124 bacts as the shaft 3123 of the larger piston 3125 a of the larger-borehydraulic cylinder 3124 a. The piston 3125 a and smaller-bore cylinder3124 b (i.e., the shaft of the larger-bore hydraulic cylinder 3124 a)are moved by the mechanical boundary mechanism 3121 in the directionindicated by the arrow 3122. Compressed hydraulic fluid from thehigher-pressure chamber 3126 a of the larger-bore cylinder 3124 a passesthrough a valve 3120 to an arrangement of check valves 3128 and thehydraulic power unit 3129, thereby producing electricity. Hydraulicfluid at a lower pressure is conducted from the output of the hydraulicpower unit through valve 3118 to the lower-pressure chamber 3116 a ofthe hydraulic cylinder 3124 a. In this state of operation, the piston3125 b of the smaller-bore cylinder 3124 b remains stationary withrespect thereto, and no fluid flows into or out of either of itschambers 3116 b, 3126 b.

Reference is now made to FIG. 31B, which shows another state ofoperation of the system 3100 of FIG. 31A. The action of the pneumaticcylinder 3101 and the direction of motion of the pistons is the same asin FIG. 31A. In FIG. 31B, the piston 3125 a and smaller-bore cylinder3124 b (i.e., shaft of the larger-bore hydraulic cylinder 3124 a) havemoved to the extreme of their ranges of motion and has stopped movingrelative to the larger-bore cylinder 3124 a. Valves are now opened suchthat the piston 3125 b of the smaller-bore cylinder 3124 b acts.Pressurized fluid from the higher-pressure chamber 3126 b of thesmaller-bore cylinder 3124 b is conducted through a valve 3133 to theaforementioned arrangement of check valves 3128 and the hydraulic powerunit 3129, thereby producing electricity. Hydraulic fluid at a lowerpressure is conducted from the output of the hydraulic power unitthrough valve 3135 to the lower-pressure chamber 3116 b of thesmaller-bore hydraulic cylinder 3124 b. In this manner, the effectivepiston area on the hydraulic side is changed during the pneumaticexpansion, narrowing the hydraulic pressure range for a given pneumaticpressure range.

Reference is now made to FIG. 31C, which shows another state ofoperation of the system 3100 of FIGS. 31A and 31B. The action of thepneumatic cylinder 3101 and the direction of motion of the pistons arethe reverse of those shown in FIG. 31A. As in FIG. 31A, only thelarger-bore hydraulic cylinder 3124 a is active. The piston 3124 b ofthe smaller bore cylinder 3124 b remains stationary, and no fluid flowsinto or out of either of its chambers 3116 b, 3126 b. Compressedhydraulic fluid from the higher-pressure chamber 3116 a of thelarger-bore cylinder 3124 a passes through a valve 3118 to theaforementioned arrangement of check valves 3128 and the hydraulic powerunit 3129, thereby producing electricity. Hydraulic fluid at a lowerpressure is conducted from the output of the hydraulic power unitthrough valve 3120 to the lower-pressure chamber 3126 a of thelarger-bore hydraulic cylinder 3124 a.

Additionally, in yet another state of operation of the system 3100, thepiston 3125 a and the smaller-bore hydraulic cylinder 3124 b (i.e., theshaft of the larger-bore hydraulic cylinder 3124 a) have moved as far asthey can in the direction indicated in FIG. 31C. Then, as in FIG. 31B,but in the opposite direction of motion, the smaller-bore hydrauliccylinder 3124 b becomes the active cylinder driving the hydraulic powerunit 3129.

It should also be clear that the principle of adding cylinders operatingat progressively lower pressures in series (pneumatic and/or hydraulic)and in parallel or telescopic fashion (mechanically) may be carried outto two or more cylinders on the pneumatic side, the hydraulic side, orboth.

Furthermore, the system 3100 shown in FIGS. 31A-31C may also include aheat-transfer subsystem 3150 similar to those described above.Generally, the heat-transfer subsystem 3150 includes a fluid circulator3152 and a heat exchanger 3154. The subsystem 3150 also includes twodirectional control valves 3156, 3158 that selectively connect thesubsystem 3150 to one or more chambers of the pneumatic cylinder 3101via pairs of gas ports on the cylinder 3101 identified as A and B. Forexample, the valves 3156, 3158 may be positioned to place the subsystem3150 in fluidic communication with chamber 3103 during gas expansiontherein, so as to thermally condition the gas expanding in the chamber3103. The gas may be thermally conditioned by any of the previouslydescribed methods. For example, during expansion (or compression), aheat-exchange liquid (e.g., water) may be drawn from a reservoir (notshown, but similar to those described above with respect to FIG. 22) bythe circulator 3154, circulated through a liquid-to-liquid version ofthe heat exchanger 3154, which may be a shell-and-tube type with aninput 3162 and an output 3160 from the shell running to an environmentalheat exchanger or to a source of process heat, cold water, or otherexternal heat exchange medium.

FIG. 32 illustrates the use of pressurized stored gas to operate adouble-acting pneumatic cylinder and linear motor/generator to produceelectricity according to another illustrative embodiment of theinvention. If the linear motor/generator is operated as a motor ratherthan as a generator, the identical mechanism employs electricity toproduce pressurized stored gas. FIG. 32 shows the mechanism beingoperated to produce electricity from stored pressurized gas.

The illustrated energy storage and recovery system 3200 includes apneumatic cylinder 3202 divided into two compartments 3204 and 3206 by apiston (or other mechanism) 3208. The cylinder 3202, which is shown in avertical orientation in FIG. 32 but may be arbitrarily oriented, has oneor more gas circulation ports 3210 (only one of which is explicitlylabeled), which are connected via piping 3212 to a compressed-gasreservoir 3214 and a vent 3216.

The piping 3212 connecting the compressed-gas reservoir 3214 tocompartments 3204, 3206 of the cylinder 3202 passes through valves 3218,3220. Compartments 3204, 3206 of the cylinder 3202 are connected to vent3216 through valves 3222, 3224. A shaft 3226 coupled to the piston 3208is coupled to one end of a translator 3228 of a linear electricmotor/generator 3230.

System 3200 is shown in two operating states, namely (a) valves 3218 and3222 open and valves 3220 and 3224 closed (shown in FIG. 32), and (b)valves 3218 and 3222 closed and valves 3220 and 3224 open (shown in FIG.33). In state (a), high-pressure gas flows from the high-pressurereservoir 3214 through valve 3218 into compartment 3204 (where it isrepresented by stippling in FIG. 32). Lower-pressure gas is vented fromthe other compartment 3206 via valve 3222 and vent 3216. The result ofthe net force exerted on the piston 3208 by the pressure differencebetween the two compartments 3204, 3206 is the linear movement of piston3208, piston shaft 3226, and translator 3228 in the direction indicatedby the arrow 3232, causing an EMF to be induced in the stator of thelinear motor/generator 3230. Power electronics are typically connectedto the motor/generator 3230, and may be software-controlled. Such powerelectronics are conventional and not shown in FIG. 32 or in subsequentfigures.

FIG. 33 shows system 3200 in a second operating state, theabove-described state (b) in which valves 3220 and 3224 are open andvalves 3218 and 3222 are closed. In this state, gas flows from thehigh-pressure reservoir 3214 through valve 3220 into compartment 3206.Lower-pressure gas is vented from the other compartment 3204 via valve3224 and vent 3216. The result is the linear movement of piston 3208,piston shaft 3226, and translator 3228 in the direction indicated by thearrow 3302, causing an EMF to be induced in the stator of the linearmotor/generator 3230.

FIG. 34 illustrates the addition of expedited heat transfer by a liquidspray as described above. In this illustrative embodiment, a spray ofdroplets of liquid (indicated by arrows 3440) is introduced into eithercompartment (or both compartments) of the cylinder 3402 throughperforated spray heads 3442, 3444, 3446, and 3448. The arrangement ofspray heads shown is illustrative only; any suitable number anddisposition of spray heads inside the cylinder 3402 may be employed.Liquid may be conveyed to spray heads 3446 and 3448 on the piston 3408by a center-drilled channel 3450 in the piston shaft 3426, and may beconveyed to spray heads 3442 and 3444 by appropriate piping (not shown).Liquid flow to the spray heads 3442, 3444, 3446, and 3448 is typicallycontrolled by an appropriate valve system (not shown).

FIG. 34 depicts system 3400 in the first of the two above-describedoperating states, where valves 3420 and 3424 are open and valves 3418and 3422 are closed. In this state, gas flows from the high-pressurereservoir 3414 through valve 3420 into compartment 3406. Liquid at atemperature higher than that of the expanding gas is sprayed (indicatedby arrows 3440) into compartment 3406 from spray heads 3442, 3444, andheat flows from the droplets 3440 to the gas. With suitable liquidtemperature and flow rate, this arrangement enables substantiallyisothermal expansion of the gas in compartment 3406.

Lower-pressure gas is vented from the other compartment 3404 via valve3424 and vent 3416, resulting in the linear movement of piston 3408,piston shaft 3426, and translator 3428 in the downward direction (arrow3452). Since the expansion of the gas in compartment 3406 issubstantially isothermal, more mechanical work is performed on thepiston 3408 by the expanding gas and more electric energy is produced bythe linear motor/generator 3430 than would be produced by adiabaticexpansion in system 3400 of a like quantity of gas.

FIG. 35 shows the illustrative embodiment of FIG. 34 in a secondoperating state, where valves 3418 and 3422 are open and valves 3420 and3424 are closed. In this state, gas flows from the high-pressurereservoir 3414 through valve 3418 into compartment 3404. Liquid at atemperature higher than that of the expanding gas is sprayed (indicatedby arrows 3440) into compartment 3404 from spray heads 3446 and 3448,and heat flows from the droplets 3440 to the gas. With suitable liquidtemperature and flow rate, this arrangement enables the substantiallyisothermal expansion of the gas in compartment 3404. Lower-pressure gasis vented from the other compartment 3406 via valve 3422 and vent 3416.The result is the linear movement of piston 3408, piston shaft 3426, andtranslator 3428 in the upward direction (arrow 3452), generatingelectricity.

System 3400 may be operated in reverse, in which case the linearmotor/generator 3430 operates as an electric motor. The droplet spraymechanism is used to cool gas undergoing compression (achievingsubstantially isothermal compression) for delivery to the storagereservoir rather than to warm gas undergoing expansion from thereservoir. System 3400 may thus operate as a full-cycle energy storagesystem with high efficiency.

Additionally, the spray-head-based heat transfer illustrated in FIGS. 34and 35 for vertically oriented cylinders may be replaced or augmentedwith a spray-rod heat transfer scheme for arbitrarily oriented cylindersas described above.

FIG. 36 is a schematic of system 3600 with the addition of expeditedheat transfer by a heat-exchange subsystem that includes an externalheat exchanger 3602 connected by piping through valves 3604, 3606 tochamber 3608 of the cylinder 3610 and by piping through valves 3612,3614 to chamber 3616 of the cylinder 3610. A circulator 3618, which ispreferably capable of pumping gas at high pressure (e.g., approximately3,000 psi), drives gas through one side of the heat exchanger 3602,either continuously or in installments. An external system, not shown,drives a fluid 3620 (e.g., air, water, or another fluid) from anindependent source through the other side of the heat exchanger.

The heat-exchange subsystem, which may include heat exchanger 3602,circulator 3618, and associated piping, valves, and ports, transfers gasfrom either chamber 3608, 3616 (or both chambers) of the cylinder 3610through the heat exchanger 3602. The subsystem has two operating states,either (a) valves 3612, 3614, 3622, and 3624 closed and valves 3604,3606, 3626, and 3628 open, or (b) valves 3612, 3614, 3622, and 3624 openand valves 3604, 3606, 3626, and 3628 closed. FIG. 36 depicts state (a),in which high-pressure gas is conveyed from the reservoir 3628 tochamber 3608 of the cylinder 3610; meanwhile, low-pressure gas isexhausted from chamber 3616 via valve 3628 to the vent 3630.High-pressure gas is also circulated from chamber 3608 through valve3604, circulator 3618, heat exchanger 3602, and valve 3606 (in thatorder) back to chamber 3608. Simultaneously, fluid 3620 warmer than thegas flowing through the heat exchanger 3602 is circulated through theother side of the heat exchanger 3602. With suitable temperature andflow rate of fluid 3620 through the external side of the heat exchanger3602 and suitable flow rate of high-pressure gas through the cylinderside of the heat exchanger 3602, this arrangement enables thesubstantially isothermal expansion of the gas in compartment 3608.

In FIG. 36, the piston shaft 3632 and linear motor/generator translator3634 are moving in the direction shown by the arrow 3636. It should beclear that, like the illustrative embodiment shown in FIG. 32, theembodiment shown in FIG. 36 has a second operating state (not shown),defined by the second of the two above-described valve arrangements(“state (b)” above), in which the direction of piston/translator motionis reversed. Moreover, this identical mechanism may clearly be operatedin reverse—in that mode (not shown), the linear motor/generator 3638operates as an electric motor and the heat exchanger 3602 cools gasundergoing compression (achieving substantially isothermal compression)for delivery to the storage reservoir 3628 rather than warming gasundergoing expansion. Thus, system 3600 may operate as a full-cycleenergy storage system with high efficiency.

FIG. 37 depicts a system 3700 that includes a second pneumatic cylinder3702 operating at a pressure lower than that of a first cylinder 3704.Both cylinders 3702, 3704 are, in this embodiment, double-acting. Theyare connected in series (pneumatically) and in line (mechanically).Pressurized gas from the reservoir 3706 drives the piston 3708 of thedouble-acting high-pressure cylinder 3704. Series attachment of the twocylinders directs gas from the lower-pressure compartment 3710 of thehigh-pressure cylinder 3704 to the higher-pressure compartment 3712 ofthe low-pressure cylinder 3702. In the operating state depicted in FIG.37, gas from the lower-pressure side 3714 of the low-pressure cylinder3702 exits through vent 3716. Through their common piston shaft 3718,the two cylinders act jointly to move the translator 3720 of the linearmotor/generator 3722. This arrangement reduces the range of pressuresover which the cylinders jointly operate, as described above.

System 3700 is shown in two operating states, (a) valves 3724, 3726, and3728 closed and valves 3730, 3732, and 3734 are open (depicted in FIG.37), and (b) valves 3724, 3726, and 3728 open and valves 3730, 3732, and3734 closed (depicted in FIG. 38). FIG. 37 depicts state (a), in whichgas flows from the high-pressure reservoir 3706 through valve 3730 intocompartment 3736 of the high-pressure cylinder 3704.Intermediate-pressure gas (indicated by stippled areas in the figure) isdirected from compartment 3710 of the high-pressure cylinder 3704 bypiping through valve 3732 to compartment 3712 of the low-pressurecylinder 3702. The force of this intermediate-pressure gas on the piston3738 acts in the same direction (i.e., in the direction indicated by thearrow 3740) as that of the high-pressure gas in compartment 3736 of thehigh-pressure cylinder 3704. The cylinders thus act jointly to movetheir common piston shaft 3718 and the translator 3720 of the linearmotor/generator 3722 in the direction indicated by arrow 3740,generating electricity during the stroke. Low-pressure gas is ventedfrom the low-pressure cylinder 3702 through the vent 3716 via valve3734.

FIG. 38 depicts state (b) of system 3700. Valves 3724, 3726, and 3728are open and valves 3730, 3732, and 3734 are closed. In this state, gasflows from the high-pressure reservoir 3706 through valve 3724 intocompartment 3710 of the high-pressure cylinder 3704.Intermediate-pressure gas is directed from the other compartment 3736 ofthe high-pressure cylinder 3704 by piping through valve 3726 tocompartment 3714 of the low-pressure cylinder 3702. The force of thisintermediate-pressure gas on the piston 3738 acts in the same direction(i.e., in direction indicated by the arrow 3742) as that of thehigh-pressure gas in compartment 3710 of the high-pressure cylinder3704. The cylinders thus act jointly to move the common piston shaft3718 and the translator 3720 of the linear motor/generator 3722 in thedirection indicated by arrow 3742, generating electricity during thestroke, which is in the direction opposite to that shown in FIG. 37.Low-pressure gas is vented from the low-pressure cylinder 3702 throughthe vent 3716 via valve 3728.

The spray arrangement for heat exchange shown in FIGS. 37 and 38 or,alternatively (or in addition to), the external heat-exchangerarrangement shown in FIG. 36 (or another heat-exchange mechanism) may bestraightforwardly adapted to the system 3700 of FIGS. 37 and 38,enabling substantially isothermal expansion of the gas in thehigh-pressure reservoir 3706. Moreover, system 3700 may be operated as acompressor (not shown) rather than as a generator. Finally, theprinciple of adding cylinders operating at progressively lower pressuresin series (pneumatic) and in line (mechanically) may involve three ormore cylinders rather than merely two cylinders as shown in theillustrative embodiment of FIGS. 37 and 38.

FIG. 39 depicts an energy storage and recovery system 3900 with a firstpneumatic cylinder 3902 and a second pneumatic cylinder 3904 operatingat a lower pressure than the first cylinder 3902. Both cylinders 3902,3904 are double-acting. They are attached in series (pneumatically) andin parallel (mechanically). Pressurized gas from the reservoir 3906drives the piston 3908 of the double-acting high-pressure cylinder 3902.Series pneumatic attachment of the two cylinders is as detailed abovewith reference to FIGS. 37 and 38. Gas from the lower-pressure side ofthe low-pressure cylinder 3904 is directed through valve 3932 to vent3910. Through a common beam (mechanical boundary mechanism) 3912 coupledto the piston shafts 3914, 3916 of the cylinders 3902, 3904, thecylinders 3902, 3904 act jointly to move the translator 3918 of thelinear motor/generator 3920. This arrangement reduces the operatingrange of cylinder pressures as compared to a similar arrangementemploying only one cylinder.

System 3900 is shown in two operating states, (a) valves 3922, 3924, and3926 closed and valves 3928, 3930, and 3932 open (shown in FIG. 39), and(b) valves 3922, 3924, and 3926 open and valves 3928, 3930, and 3932closed (shown in FIG. 40). FIG. 39 depicts state (a), in which gas flowsfrom the high-pressure reservoir 3906 through valve 3928 intocompartment 3934 of the high-pressure cylinder 3902.Intermediate-pressure gas (depicted by stippled areas) is directed fromthe other compartment 3936 of the high-pressure cylinder 3902 by pipingthrough valve 3930 to compartment 3938 of the low-pressure cylinder3904. The force of this intermediate-pressure gas on the piston 3940acts in the same direction (i.e., in direction indicated by the arrow3942) as the high-pressure gas in compartment 3934 of the high-pressurecylinder 3902. The cylinders thus act jointly to move the common beam3912 and the translator 3918 of the linear motor/generator 3920 in thedirection indicated by arrow 3942, generating electricity during thestroke. Low-pressure gas is vented from the low-pressure cylinder 3904through the vent 3910 via valve 3932.

FIG. 40 shows the second operating state (b) of system 3900, i.e.,valves 3922, 3924, and 3926 are open and valves 3928, 3930, and 3932 areclosed. In this state, gas flows from the high-pressure reservoir 3906through valve 3922 into compartment 3936 of the high-pressure cylinder3902. Intermediate-pressure gas is directed from compartment 3934 of thehigh-pressure cylinder 3902 by piping through valve 3924 to compartment3944 of the low-pressure cylinder 3904. The force of thisintermediate-pressure gas on the piston 3940 acts in the same direction(i.e., in direction indicated by the arrow 3942) as that exerted onpiston 3908 by the high-pressure gas in compartment 3936 of thehigh-pressure cylinder 3902. The cylinders 3902, 3904 thus act jointlyto move the common beam 3912 and the translator 3918 of the linearmotor/generator 3920 in the direction indicated, generating electricityduring the stroke, which is in the direction opposite to that of theoperating state shown in FIG. 39. Low-pressure gas is vented from thelow-pressure cylinder 3904 through the vent 3910 via valve 3926.

The spray arrangement for heat exchange shown in FIGS. 34 and 35 or,alternatively or in combination, the external heat-exchanger arrangementshown in FIG. 36 may be straightforwardly adapted to the pneumaticcylinders of system 3900, enabling substantially isothermal expansion ofthe gas in the high-pressure reservoir 3906. Moreover, this exemplaryembodiment may be operated as a compressor (not shown) rather than agenerator (shown). Finally, the principle of adding cylinders operatingat progressively lower pressures in series (pneumatic) and in parallel(mechanically) may be extended to three or more cylinders.

FIG. 41 is a schematic diagram of a system 4100 for achievingsubstantially isothermal compression and expansion of a gas for energystorage and recovery using a pair of pneumatic cylinders (shown inpartial cross-section) with integrated heat exchange. In thisillustrative embodiment, the mechanism linking the cylinders convertsreciprocal motion of the cylinders to rotary motion. Depicted are a pairof double-acting pneumatic cylinders with appropriate valving andmechanical linkages; however, any number of single- or double-actingpneumatic cylinders, or any number of groups of single- or double-actingpneumatic cylinders, where each group contains two or more cylinders,may be employed in such a system. Likewise, a wrist-pin connecting-rodtype crankshaft arrangement is depicted in FIG. 41, but other mechanicalmeans for converting reciprocal motion to rotary motion are contemplatedand considered within the scope of the invention.

In various embodiments, the system 4100 includes a first pneumaticcylinder 4102 divided into two compartments 4104, 4106 by a piston 4108.The cylinder 4102, which is shown in a vertical orientation in thisillustrative embodiment, has one or more ports 4110 (only one of whichis explicitly labeled) that are connected via piping 4112 to acompressed-gas reservoir 4114.

The system 4100 as shown in FIG. 41 includes a second pneumatic cylinder4116 operating at a lower pressure than the first cylinder 4102. Thesecond pneumatic cylinder 4116 is divided into two compartments 4118,4120 by a piston 4122 and includes one or more ports 4110 (only one ofwhich is explicitly labeled). Both cylinders 4102, 4116 aredouble-acting in this illustrative embodiment. They are attached inseries (pneumatically); thus, after expansion in one compartment of thehigh-pressure cylinder 4102, the mid-pressure gas (depicted by stippledareas) is directed for further expansion to a compartment of thelow-pressure cylinder 4116.

In the state of operation depicted in FIG. 41, pressurized gas (e.g.,approximately 3,000 psig) from the reservoir 4114 passes through a valve4126 and drives the piston 4108 of the double-acting high-pressurecylinder 4102 in the downward direction as shown by the arrow 4128. Gasthat has already expanded to a mid-pressure (e.g., approximately 250psig) in the lower chamber 4104 of the high-pressure cylinder 4102 isdirected through a valve 4130 to the lower chamber 4118 of thelarger-volume, low-pressure cylinder 4116, where it is further expanded.This gas exerts an upward force on the piston 4122 with resulting upwardmotion of the piston 4122 and shaft 4130 as indicated by the arrow 4132.Gas within the upper chamber 4120 of cylinder 4116 has already beenexpanded to atmospheric pressure and is vented to the atmosphere throughvalve 4134 and vent 4136. One function of this two-cylinder arrangementis to reduce the range of pressures and forces over which each cylinderoperates, as described earlier.

The piston shaft 4138 of the high-pressure cylinder 4102 is connected bya hinged connecting rod 4140 and crank 4146 or other suitable linkage toa crankshaft 4142. The piston shaft 4130 of the low-pressure cylinder4116 is connected by a hinged connecting rod 4144 and crank 4148 orother suitable linkage to the same crankshaft 4142. The motion of thepiston shafts 4130, 4138 is shown as rectilinear, whereas the linkages4140, 4144 have partial rotational freedom orthogonal to the axis of thecrankshaft 4142.

In the state of operation shown in FIG. 41, the piston shaft 4138 andlinkage 4140 are drawing the crank 4146 in a downward direction (asindicated by arrow 4128) while the piston shaft 4130 and linkage 4144are pushing the crank 4148 in an upward direction (as indicated by arrow4132). The two cylinders 4102, 4116 thus act jointly to rotate thecrankshaft 4142. In FIG. 41, the crankshaft 4142 is shown driving anoptional transmission mechanism 4150 whose output shaft 4152 rotates ata higher rate than the crankshaft 4142. Transmission mechanism 4150 maybe, e.g., a gear box or a CVT (as shown in FIG. 41). The output shaft4152 of transmission mechanism 4150 drives an electric motor/generator4154 that generates electricity. In some embodiments, crankshaft 4142 isdirectly connected to and drives motor/generator 4154.

Power electronics may be connected to the motor/generator 4154 (and maybe software-controlled), thus providing control over air expansionand/or compression rates. These power electronics are not shown, but arewell-known to a person of ordinary skill in the art.

In the embodiment of the invention depicted in FIG. 41, liquid spraysmay be introduced into any of the compartments of the cylinders 4102,4116. In both cylinders 4102, 4116, the liquid spray enables expeditedheat transfer to (or from) the gas being expanded (or compressed) in thecylinder, as detailed above. Sprays 4156, 4158 of droplets of liquid maybe introduced into the compartments of the high-pressure cylinder 4102through perforated spray heads 4160, 4162. The liquid spray in chamber4106 of cylinder 4102 is indicated by dashed lines 4158, and the liquidspray in chamber 4104 of cylinder 4102 is indicated by dashed lines4156. Water (or other appropriate heat-transfer fluid) is conveyed tothe spray heads 4162 by appropriate piping (not shown). Fluid may beconveyed to spray head 4160 on the piston 4108 by various methods; inone embodiment, the fluid is conveyed through a center-drilled channel(not shown) in the piston rod 4138, as described in U.S. patentapplication Ser. No. 12/690,513 (the '513 application), the disclosureof which is hereby incorporated by reference herein in its entirety.Liquid flow to both sets of spray heads is typically controlled by anappropriate valve arrangement (not shown). Liquid may be removed fromthe cylinders through suitable ports (not shown).

The heat-transfer liquid sprays 4156, 4158 may warm gas as it expands,enabling substantially isothermal expansion of the gas. If the gas isbeing compressed, the sprays may cool the gas, enabling substantiallyisothermal compression. A liquid spray may be introduced by similarmeans into the compartments of the low-pressure cylinder 4116 throughperforated spray heads 4164, 4166. Liquid spray in chamber 4118 ofcylinder 4116 is indicated by dashed lines 4168.

In the operating state shown in FIG. 41, liquid spray transfers heat to(or from) the gas undergoing expansion (or compression) in chambers4104, 4106, and 4118, enabling a substantially isothermal process. Spraymay be introduced in chamber 4120, but this is not shown as little or noexpansion is occurring in that compartment during venting. Thearrangement of spray heads shown in FIG. 41 is illustrative only, as anynumber and disposition of spray heads and/or spray rods inside thecylinders 4102, 4116 are contemplated as embodiments of the presentinvention.

FIG. 42 depicts system 4100 in a second operating state, in which thepiston shafts 4130, 4138 of the two pneumatic cylinders 4102, 4116 havedirections of motion opposite to those shown in FIG. 41, and thecrankshaft 4142 continues to rotate in the same sense as in FIG. 41. InFIG. 42, valves 4124, 4130, and 4134 are closed and valves 4126, 4170,and 4172 are open. Gas flows from the high-pressure reservoir 4114through valve 4126 into compartment 4104 of the high-pressure cylinder4102, where it applies an upward force on piston 4108. Mid-pressure gasin chamber 4106 of the high-pressure cylinder 4102 is directed throughvalve 4170 to the upper chamber 4120 of the low-pressure cylinder 4116,where it is further expanded. The expanding gas exerts a downward forceon the piston 4122 with resulting motion of the piston 4122 and shaft4130 as indicated by the arrow 4132. Gas within the lower chamber 4118of cylinder 4116 is already expanded to approximately atmosphericpressure and is being vented to the atmosphere through valve 4172 andvent 4136. In FIG. 42, gas expanding in chambers 4104, 4106, and 4120exchanges heat with liquid sprays 4156, 4158, and 4174 (depicted asdashed lines), respectively, to keep the gas at approximately constanttemperature.

The spray-head heat-transfer arrangement shown in FIGS. 41 and 42 forvertically oriented cylinders may be replaced or augmented with aspray-rod heat-transfer scheme for arbitrarily oriented cylinders (asmentioned above). Additionally, the systems shown may be implementedwith an external gas heat exchanger instead of (or in addition to)liquid sprays, as described above. An external gas heat exchanger alsoenables expedited heat transfer to or from the gas being expanded (orcompressed) in the cylinders. With an external heat exchanger, thecylinders may be arbitrarily oriented.

In all operating states, the two cylinders 4102, 4116 in FIGS. 41 and 42are preferably 180° out of phase. For example, whenever the piston 4108of the high-pressure cylinder 4102 has reached its uppermost point ofmotion, the piston 4122 of the low-pressure cylinder 4116 has reachedits nethermost point of motion. Similarly, whenever the piston 4122 ofthe low-pressure cylinder 4116 has reached its uppermost point ofmotion, the piston 4108 of the high-pressure cylinder 4102 has reachedits nethermost point of motion. Further, when the two pistons 4108, 4122are at the midpoints of their respective strokes, they are moving inopposite directions. This constant phase relationship is maintained bythe linkage of the piston rods 4130, 4138 to the two cranks 4146, 4148,which are affixed to the crankshaft 4142 so that they lie in a singleplane on opposite sides of the crankshaft 4142 (i.e., they arephysically 180° apart). At the moments depicted in FIG. 41 and FIG. 42,the plane in which the two cranks 4146, 4148 lie are coincident with theplanes of the figures.

Reference is now made to FIG. 43, which is a schematic depiction of asingle pneumatic cylinder assembly 4300 and a mechanical linkage thatmay be used to connect the rod or shaft 4302 of the cylinder assembly toa crankshaft 4304. Two orthogonal views of the linkage and piston areshown in partial cross section in FIG. 43. In this illustrativeembodiment, the linkage includes a crosshead 4306 mounted on the end ofthe rod 4302. The crosshead 4306 is slidably disposed within a distancepiece 4308 that constrains the lateral motion of the crosshead 4306. Thedistance piece 4308 may also fix the distance between the top of thecylinder 4310 and a housing (not depicted) of the crankshaft 4304.

A connecting pin 4312 is mounted on the crosshead 4306 and is free torotate around its own long axis. A connecting rod 4314 is attached tothe connecting pin 4312. The other end of the connecting rod 4314 isattached to a collar-and-pin linkage 4316 mounted on a crank 4318affixed to the crankshaft 4304. A collar-and-pin linkage 4314 isillustrated in FIG. 43, but other mechanisms for attaching theconnecting rod 4314 to the crank 4318 are contemplated withinembodiments of the invention. Moreover, either or both ends of thecrankshaft 4316 may be extended to attach to further cranks (not shown)interacting with other cylinders or may be linked to a gear box (orother transmission mechanism such as a CVT), motor/generator, flywheel,brake, or other device(s).

The linkage between cylinder rod 4302 and crankshaft 4316 depicted inFIG. 43 is herein termed a “crosshead linkage,” which transformssubstantially rectilinear mechanical force acting along the cylinder rod4302 into torque or rotational force acting on the crankshaft 4316.Forces transmitted by the connecting rod 4302 and not acting along theaxis of the cylinder rod 4316 (e.g., lateral forces) act on theconnecting pin 4312, crosshead 4306, and distance piece 4308 but not onthe cylinder rod 4302. Thus, advantageously, any gaskets or seals (notdepicted) through which the cylinder rod 4302 slides while passing intocylinder 4310 are subject to reduced stress, enabling the use of lessdurable gaskets or seals, increasing the lifespan of the employedgaskets or seals, or both.

FIGS. 44A and 44B are schematics of a system 4400 for substantiallyisothermal compression and expansion of a gas for energy storage andrecovery using multiple pairs 4402 of pneumatic cylinders withintegrated heat exchange. Storage of compressed air, venting oflow-pressure air, and other components of the system 4400 are notdepicted in FIGS. 44A and 44B, but are consistent with the descriptionsof similar systems herein. Each rectangle in FIGS. 44A and 44B labeledPAIR 1, PAIR 2, etc. represents a pair of pneumatic cylinders (withappropriate valving and linkages, not explicitly depicted) similar tothe pair of cylinders depicted in FIG. 41. Each cylinder pair 4402 is apair of fluidly linked pneumatic cylinders communicating with a commoncrankshaft 4404 by a mechanism that may resemble those shown in FIG. 41or FIG. 43 (or may have some other form). The crankshaft 4404 maycommunicate (with or without an intervening transmission mechanism) withan electric motor/generator 4406 that may thus generate electricity.

In various embodiments, within each of the cylinder pairs 4402 shown inFIGS. 44A and 44B, the high-pressure cylinder (not explicitly depicted)and the low-pressure cylinder (not explicitly depicted) are 180° out ofphase with each other, as depicted and described for the two cylinders4102, 4116 in FIG. 41. For simplicity, the phase of each cylinder pair4402 is identified herein with the phase of its high-pressure cylinder.In the embodiment depicted in FIG. 44A, which includes six cylinderpairs 4402, the phase of PAIR 1 is arbitrarily denoted 0°. The phase ofPAIR 2 is 120°, the phase of PAIR 3 is 240°, the phase of PAIR 4 is 360°(equivalent to 0°), the phase of PAIR 5 is 120°, and the phase of PAIR 6is 240°. There are thus three sets of cylinder pairs 4402 that are inphase, namely PAIR 1 and PAIR 4 (0°), PAIR 2 and PAIR 5 (120°), and PAIR3 and PAIR 6 (240°). These phase relationships are set and maintained bythe affixation to the crankshaft 4404 at appropriate angles of thecranks (not explicitly depicted) linked to each of the cylinders in thesystem 1300.

In the embodiment depicted in FIG. 44B, which includes four cylinderpairs 4402, the phase of PAIR 1 is also denoted 0°. The phase of PAIR 2is then 270°, the phase of PAIR 3 is 90°, and the phase of PAIR 4 is180°. As in FIG. 44A, these phase relationships are set and maintainedby the affixation to the crankshaft 4404 at appropriate angles of thecranks linked to each of the cylinders in the system 4400.

Linking an even number of cylinder pairs 4402 to a single crankshaft4404 advantageously balances the forces acting on the crankshaft:unbalanced forces generally tend to either require more durable parts orshorten component lifetimes. An advantage of specifying the phasedifferences between the cylinder pairs 4402 as shown in FIGS. 44A and44B is minimization of fluctuations in total force applied to thecrankshaft 4402. Each cylinder pair 4402 applies a force varying betweenzero and some maximum value (e.g., approximately 330,000 lb) during thecourse of a single stroke. The sum of all the torques applied by themultiple cylinder pairs 4402 to the crankshaft 4404 as arranged in FIGS.44A and 4B varies by less than the torque applied by a single cylinderpair 4402, both absolutely and as a fraction of maximum torque, and istypically never zero.

Generally, the systems described herein may be operated in both anexpansion mode and in the reverse compression mode as part of afull-cycle energy storage system with high efficiency. For example, thesystems may be operated as both compressor and expander, storingelectricity in the form of the potential energy of compressed gas andproducing electricity from the potential energy of compressed gas.Alternatively, the systems may be operated independently as compressorsor expanders.

In addition, the systems described above, and/or other embodimentsemploying liquid-spray heat exchange or external gas heat exchange (asdetailed above), may draw or deliver thermal energy via theirheat-exchange mechanisms to external systems (not shown) for purposes ofcogeneration, as described in the '513 application.

Having described certain embodiments of the invention, it will beapparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. The terms andexpressions employed herein are used as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding any equivalents of the features shown anddescribed or portions thereof, but it is recognized that variousmodifications are possible within the scope of the invention claimed.

What is claimed is: 1.-20. (canceled)
 21. A method of energy recovery,the method comprising: introducing compressed gas at a first pressureinto a first cylinder assembly comprising a piston therewithin;expanding the gas within the first cylinder assembly to a secondpressure lower than the first pressure, thereby translating the pistonwithin the first cylinder assembly; converting the translation of thepiston into rotary motion of a motor/generator outside the firstcylinder assembly; introducing heat-transfer fluid into the gas suchthat the heat-transfer fluid exchanges heat with the gas within thefirst cylinder assembly during expansion; controlling operation of thefirst cylinder assembly (i) to enforce substantially isothermalexpansion of gas therein and (ii) in response to at least one systemparameter associated with operation of the first cylinder assembly; andventing substantially all of the expanded gas at the second pressurefrom the first cylinder assembly to atmosphere.
 22. The method of claim21, wherein the translation of the piston is converted into rotarymotion by a transmission mechanism.
 23. The method of claim 22, whereinthe transmission mechanism comprises a crankshaft.
 24. The method ofclaim 22, wherein the transmission mechanism comprises a crankshaft anda gear box.
 25. The method of claim 22, wherein the transmissionmechanism comprises a crankshaft and a continuously variabletransmission.
 26. The method of claim 22, wherein the transmissionmechanism is connected to the piston by a shaft and a crosshead linkage.27. The method of claim 22, wherein the transmission mechanism variestorque for a given force exerted thereon.
 28. The method of claim 21,wherein the at least one system parameter comprises at least one of afluid state, a fluid flow, a temperature, or a pressure.
 29. The methodof claim 21, wherein the at least one system parameter comprises aposition of the piston within the first cylinder assembly.
 30. Themethod of claim 21, wherein the at least one system parameter ismonitored by at least one sensor.
 31. The method of claim 21, furthercomprising, prior to introducing gas into the first cylinder assembly,expanding gas from a third pressure higher than the first pressure toapproximately the first pressure within a second cylinder assembly (i)comprising a piston therewithin and (ii) selectively fluidly connectedto the first cylinder assembly.
 32. The method of claim 31, furthercomprising, prior to expanding gas within the second cylinder assembly,transferring gas from a compressed-gas reservoir to the second cylinderassembly at approximately the third pressure.
 33. The method of claim31, wherein the piston within the first cylinder assembly and the pistonwithin the second cylinder assembly are mechanically coupled to acrankshaft.
 34. The method of claim 31, further comprising (i) afterexpanding gas from the third pressure to the first pressure within thesecond cylinder assembly, expanding one or more additional portions ofgas each from a pressure less than the third pressure to the firstpressure within the second cylinder assembly, and (ii) maintaining asubstantially constant power output of the motor/generator during theexpansions notwithstanding the different initial pressures of gas. 35.The method of claim 21, wherein gas at the first pressure is introducedinto the first cylinder assembly from a compressed-gas reservoir storinggas at approximately the first pressure.
 36. The method of claim 21,further comprising (i) after expanding gas from the first pressure tothe second pressure within the first cylinder assembly, expanding one ormore additional portions of gas each from a pressure less than the firstpressure to the second pressure within the first cylinder assembly, and(ii) maintaining a substantially constant power output of themotor/generator during the expansions notwithstanding the differentinitial pressures of gas.
 37. The method of claim 21, further comprisingmonitoring a temperature of the gas during expansion of the gas.
 38. Themethod of claim 21, further comprising monitoring a pressure of the gasduring expansion of the gas.
 39. The method of claim 21, furthercomprising monitoring, during expansion of the gas, at least one of aposition or a rate of movement of the piston.
 40. The method of claim21, wherein introducing heat-transfer fluid into the gas comprisesspraying heat-transfer fluid into the gas.